Why did the Celtics choose Joe Mazzulla to lead them out of crisis? It starts with his journey.

Assistant coach Joe Mazzulla was not well-known outside of basketball’s nerve center. The Johnston, R.I., native sat behind the Celtics bench last season, alongside video coordinators and medical staffers. He never played in the NBA, and his only head-coaching experience came at Division 2 Fairmont State, where he cleaned up soda cups and candy wrappers after games.

But Celtics brass viewed Mazzulla as a future star. His competitive fire, basketball acumen, and maniacal preparation were obvious.

At Fairmont, Mazzulla asked his wife, Camai, if they could buy an RV and live in the school gym’s parking lot, for easy access. He once arrived one minute late to a training session and punished himself with a workout so fierce it made his players worry about his safety. He combed through rule books and ended up explaining new guidelines to referees.

“We always knew Joe was too good to be at a Division 2 college,” former Fairmont forward Vince Franklin said. “His brain was just working way too much for that.”

In June 2021, Mazzulla reached the final round of interviews to be Stevens’s replacement as Celtics head coach before Udoka was eventually chosen. But just three months after leading the Celtics to the NBA Finals, Udoka was being suspended for one year, and his future beyond that is murky.

Grousbeck insists that if there had been any doubt about Mazzulla’s candidacy, a nationwide search for an interim coach would have been conducted. Too much was on the line this season.

But after Stevens contacted the 34-year-old assistant, he did not even interview him again.

“He just told me he believed in me, and that I was ready,” Mazzulla said. “And I just said, ‘Let’s do it.’ ”

Before Rob McClanaghan became a skills trainer for NBA stars such as Stephen Curry and Kevin Durant, he was a physical education teacher and assistant basketball coach at Bishop Hendricken, a private, all-boys school in Warwick, R.I. Mazzulla, a freshman in 2003, was essentially his first client.

The two conducted 6 a.m. workouts a few times a week. Sometimes Mazzulla would appear in McClanaghan’s office during bathroom breaks wearing his shirt, tie, and dress shoes, and ask to shoot baskets. Then he’d return to class 10 minutes later drenched in sweat. Joe’s mother, Latresa, would pick him up after practice each night and end up waiting another hour, because he would not leave the court.

Mazzulla, a two-time state Player of the Year, accepted a scholarship to West Virginia. When Bob Huggins became coach during Mazzulla’s sophomore year, he wanted to use the 1-3-1 defense the Mountaineers thrived with the previous season.

“And he basically had Joe install it,” said former assistant Jerrod Calhoun, chuckling. “Hugs would just hand him the white board.”

Mazzulla missed most of the following season after suffering a left shoulder injury. He tried to stay involved by taking on coaching duties but admits he was in a dark place. In April 2009, Mazzulla was arrested and charged with domestic battery for allegedly putting his hand on the neck of a woman at a Morgantown bar. He paid a $100 fine and was ordered to do 100 hours of community service.

“There are points in your life that you don’t have an identity,” Mazzulla said. “At that time I was going through a lot and just didn’t have a compass and know who I was. I really had to work on myself.”

Mazzulla met with a university counselor twice a week and completed community service by working at local basketball camps and speaking with troubled teenagers about his experiences.

He helped the Mountaineers to the Final Four in 2010 and could have carved out a career overseas. But when he trained with other pro hopefuls, he relished helping them more than working on his own game.

Mazzulla became an unpaid assistant at Division 2 Glenville State in rural West Virginia, where he and associate head coach Rob Summers took turns driving the van to road games. They booked motels, washed uniforms, and cooked team meals.

“We’d usually make spaghetti,” Summers said, “and make sure we didn’t burn the place down.”

Mazzulla lived in a trailer park, but he was not seeking luxury in Glenville. He studied film constantly and searched for unique strategies and drills. In his spare time, he edited clips of McClanaghan’s NBA clients for extra experience and cash.

Soon after arriving in Glenville, Mazzulla went to an athletic department breakfast organized by the women’s volleyball coach, Camai Roberson, but no one else came. The two started dating, got engaged, and started planning their way out.

In 2013, Mazzulla became an assistant at Fairmont State, where he studied film with players at 7 a.m. and designed workout programs for each one. Once, when a recruit was wavering about his decision, Mazzulla found his barber and talked him into promoting Fairmont during his next haircut. It worked.

Before each practice, Mazzulla would stroll around the gym with one player and talk about classes and family and college pressures. At the end, the player had to identify something for which he was grateful.

“We viewed him as this older guy who had everything figured out,” former Fairmont guard Shammgod Wells said. “And he’d say ‘Nah, I’m the same as you. Here’s what I’m dealing with.’ It was like having a coach and a therapist.”

Mazzulla and Northern Kentucky assistant Ron Nored, a former Celtics assistant, became friends on the recruiting trail, and when Nored was hired to coach the Nets’ G League affiliate in 2017, he added Mazzulla to his staff.

But days before the season’s start, the Nets said Nored could not pick his assistants. Mazzulla had already quit his job at Fairmont, so he took a job working 9 p.m. to 3 a.m. as a barback.

“Cleaning toilets and everything,” Mazzulla said.

But on quiet nights, he brought his laptop and began truly studying the NBA.

Nored remained close to the Celtics and asked Maine Red Claws general manager Dave Lewin to hire Mazzulla to coach on Scott Morrison’s staff. After receiving glowing reviews, Lewin offered Mazzulla $15,000 and a hotel room for the G League season. Mazzulla stretched out the salary by saving most of his per diem money.

“Some coaches know X’s and O’s, and some are really good with building relationships, but very few can do both,” Morrison said. “Joe showed he could do both right out of the gate.”

After one season, Mazzulla returned to Fairmont as head coach. When he and Camai were looking for a home, he told her he wanted to buy an RV and keep it parked outside the gym.

“And when I say he was dead serious, he was dead serious,” Camai said. “He’s like, ‘We’d have free electricity. We can take showers in the locker room and do laundry in the team laundry room.’ I’m like, ‘Have you lost your mind?’ ”

The couple bought a foreclosed home instead, but Mazzulla essentially lived at the basketball facility anyway. He spent so much time studying film that Wells, who became his assistant, made him watch one hour of regular television in his office each night. Then work would continue for several more hours.

Mazzulla studied the NCAA rule book as if he were preparing for a test. During one game, Fairmont was on defense and clinging to a lead in the final seconds, and the opponent was out of timeouts. Mazzulla instructed a player to step over the end line as things went in motion, causing a delay of game warning. This gave Fairmont a peek at what was coming, and it smothered the play.

Mazzulla usually spent the first few minutes of each game silently studying the opponent’s approach. Then he would summon his point guard.

“OK,” he’d say, “here’s how things are going to go tonight.”

Once, Mazzulla was scheduled to conduct a workout with two of his players but showed up one minute late because he was meeting with the family of a recruit. He apologized profusely, but that was not enough. So he put himself through a military-style workout as punishment. He attempted an eight-minute pull-up hold, did pushups over the next eight minutes, and then completed eight minutes of sprints. By the end, he was dazed.

“We were telling him, ‘Coach, you’re good, man, chill,’ ” forward Jason Jolly said.

But Mazzulla, who donated a portion of his modest salary to build the school’s new weight room, wanted to lead by example. He wanted his players to develop into men. He’d point out that many of them would be the first in their families to graduate college. He made them recite their goals and dreams to the team, partly to speak them into existence, but mostly to help them become comfortable with public speaking.

When Franklin was hired by an automobile company after graduating in 2019, he had no way to get to work and feared he couldn’t keep the job. So Mazzulla lent him his minivan for several weeks.

“He just does a very good job of showing you that he cares,” Wells said.

Joe and Camai Mazzulla have a prayer board hanging in their home. They pin note cards and small sheets of paper listing their hopes for themselves and those close to them.

Currently, the board includes notes for current and former Celtics Mazzulla worked closely with such as Kemba Walker (identity, what’s next), Romeo Langford (direction, commitment), and Robert Williams (recovery from surgery).

Years ago, Mazzulla scrawled his own dream on the board: NBA coach. But one day Camai noticed that it was gone. Mazzulla told her he took it down because he was happy at Fairmont, happy helping people who needed him.

“Put that back up there,” she told him. “Your impact in the NBA could be even bigger.”

Former Celtics president of basketball operations Danny Ainge was impressed by Mazzulla’s stint in Maine and recruited him for a player development role after his first season leading Fairmont, but Mazzulla did not want to abandon the players he’d recruited. A year later, Stevens offered him a position on his staff. Mazzulla was still conflicted.

“His biggest weakness, as well as his strength, is his loyalty,” Camai said. “He’ll plant himself somewhere and not want to leave. He was going to try to sign, like, a 10-year deal at Fairmont.”

Mazzulla called some players and staffers to his house and sought advice. He told them he was leaning toward staying. They were furious that he was even asking.

“You have to go,” they told him. “The NBA is where you belong.”

Even after joining the Celtics, Mazzulla continued to watch Fairmont’s game film and send his former players advice. He would not let the bonds fray.

Mazzulla’s father, Dan, was a local basketball star who played professionally in Chile before becoming director of Johnston’s parks department and a successful high school coach. (Before Johnston High resurfaced its court a few years ago, Dan’s dress-shoe scuff marks, a result of his stomping, were fossilized on the sidelines.)

Joe always had a key to a gym, and his father put him through intense basketball workouts. He’d tell him not to end weightlifting sessions until he actually looked and felt stronger.

“We did pushups for everything,” Mazzulla says, smiling. “Make a mistake? Pushups. Have something good to celebrate? Pushups.”

Dan used to drive through the night from Rhode Island to watch Joe play for the Mountaineers, and now his son was going to be an assistant for the Celtics, essentially the hometown team. It was perfect.

But about a week after Joe accepted the job, Dan was diagnosed with brain cancer. Joe was crushed, but also thankful to be back in New England. Dan Mazzulla attended a few Celtics games early that season and saw his son on the sidelines in green. The NBA shutdown due to the COVID-19 pandemic gave Mazzulla more time to be with his father.

“I don’t want to say we had to give him permission to die, but we almost had to be like, ‘Hey, we got this,’ ” Mazzulla said. “He was fighting it for a little while because he didn’t want to let us down. But we could tell him, ‘You did your job.’ Once we were able to give him that peace, he was able to accept that.”

Dan Mazzulla died April 22, 2020. When Joe gave the eulogy, he encouraged everyone to be motivated by his father’s life. He had preached a hard combination of intensity, tough love, and relentlessness, but his mortality made him adjust priorities. It was the final lesson he passed to Joe.

“He told him, ‘You have to know when to ease up sometimes,’ ” Camai Mazzulla said. " ‘I taught you how to work hard, but you have to also find that joy to live while working.’ ”

Mazzulla is still finding a balance. He has a dry-erase board hanging at his Newton home, and after his 12-year-old stepson, Mikey, or 13-year-old nephew, Jaziah, play youth-league games, he sometimes cannot resist taking them there for tutorials.

“He hits you with it all,” said Mazzulla’s younger sister, Gianna. “He writes and erases, writes and erases.”

As a Celtics assistant the past three seasons, it was easier to find time. Mazzulla loves taking his 6-year-old son, Manny, and Mikey to and from school. But during his chaotic first week in this top job, he wasn’t able to do that.

“It broke my heart seeing him,” Camai said. “Joe’s not super emotional, and it was the first time in a while I’ve seen him emotional. He was like, ‘I thought we had more time.’ "

After a practice earlier this month, Mazzulla was finishing lunch at a Brighton restaurant when he said he had to leave to take Mikey to football practice. He polished off a bottle of sparkling water and crammed leftover slices of spicy sausage pizza into a to-go box for his stepson. Then he walked down the street amid young professionals who seemed unaware that they were passing a man who had recently been elevated to one of the most prestigious jobs in the city.

Mazzulla, for his part, is unfazed by flash. His brother, Justin, says he still likes to crash on friends’ couches when he returns to Rhode Island, and he made headlines in his first week as coach when he revealed he still drives the 2017 minivan he once let his players borrow.

“When I saw him say that, I said, ‘Oh, Joe, I know you’re going to drive it, but just make sure there’s no French fries or something on the floor now,’ ” his mother said. “ ‘Just in case someone looks over.’ ”

The Celtics, who are off to a 3-1 start, appear unaffected by the sudden coaching change. They worked with Mazzulla for three seasons as an assistant and have gushed about how he has taken to this new role.

Mazzulla relishes the opportunities to be in the mix, just as he did while coaching in West Virginia. He’ll hop into drills with Jayson Tatum and Jaylen Brown, and he continues to seek an edge wherever he can. He recently read about the benefits of intermittent sprinting, for example.

“So I’ll hear ‘thump, thump, thump’ at our house and I’m like, ‘What is this guy doing now?!’ ” Camai said. “And he’s doing high-knee stepping to train for the random times to do sprints.”

For the Mazzulla family, this is all still surreal. When Joe could finally say something about getting the job, he let his two siblings know with a simple text message the night before it was announced: “Interim coach. LOL.”

Mazzulla’s mother, brother, and sister still live in Rhode Island and are grateful they can witness his ascension. Mazzulla recently organized a family-style dinner for the team’s staff and their families at Strega in the North End, and he was grateful that his family was there, too.

Before the season opener against Philadelphia, Gianna Mazzulla looked down at the court and pinched herself.

“I was just like, ‘This is sick,’ ” she said. “My brother’s really coaching the Celtics. He blinked, and he’s where he always dreamed he’d be.”

It is crushing that Dan Mazzulla is not here to see it. Justin Mazzulla said their father would be so proud if he could see Joe now. And he’d continue to offer tough love, as only a Mazzulla can.

“My dad would call him every week and just be like, ‘Hey, you’ve got a lot of work to do up there,’ ” Justin Mazzulla said. “He always expected more, and that’s what drove us, especially my brother.”

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.