Red Sox lefthander Brandon Walter, expected to make his big league debut Thursday, defied the odds to reach majors

It required a degree of hardheadedness for Walter to forge a big league path. He underwent Tommy John surgery at the University of Delaware and was throwing just 86-90 miles per hour when he returned in 2019. Still, Reed Gragnani — then a Sox area scout — believed in Walter’s ability to get bad contact on his sinker and to miss bats with his changeup.

Walter signed for $35,000, reported to Fort Myers, Fla., and went to work.

“I’ve said the whole time, if they’re going to keep giving me the ball, keep giving me a jersey, I can prove some people wrong who maybe skipped me in that draft,” said Walter. “I never looked at myself as below anybody else because I got drafted later.”

His first minor league season in 2020 was wiped out by the COVID-19 pandemic. But when the Sox sent all of their minor leaguers home, Walter committed to attacking what he identified as his shortcomings.

“I knew what my strengths were before that. I knew what my weaknesses were,” said Walter. “My weakness were my velocity and just overall health of my arm and body. I wanted to get that addressed first. I think the biggest thing with that was just putting on some weight, being able to sustain the workload of a pitcher. With the weight came along with some extra ticks of velocity.”

By the spring of 2021, Walter shocked Sox officials when he reported to camp throwing his sinker in the mid-90s along with a changeup and slider from a low three-quarters arm slot that baffled hitters in the lower levels of the minors. He dominated out of the bullpen in Single A Salem and High A Greenville, ultimately forcing the Sox to look at him as a starter, a role in which he continued to dominate.

By the start of 2022, despite a drop in velocity (88-92 m.p.h. on his sinker) he had put himself on the big league radar, especially after forging a 2.88 ERA with 68 strikeouts and just three walks in 50 innings in Double A Portland.

He was promoted to Triple A Worcester by the end of May, but started just twice before being shut down with what proved to be a bulging disk in his upper back. He didn’t pitch again last season. Still, he’d shown enough to make the Sox’ decision to add him to the 40-man roster last offseason an obvious one, and to put the big leagues squarely on his radar.

“Definitely coming into this year, for sure, one of my goals was to get here and contribute,” said Walter. “[And] not only to get here but to make an impact, earn the respect of the guys in the clubhouse and the staff here and the fans obviously, to the point where they trust me to give me the ball in any situation.

As he’s been working into form following last year’s long layoff, Walter’s Triple A season hasn’t been seamless in the pursuit of that goal. He is 1-5 with a 6.84 ERA, a modest 21 percent strikeout rate, and surprisingly high 10 percent walk rate.

However, he’s shown recent evidence of improvement, with a 3.48 ERA, 13 strikeouts, and just two walks in his last two starts (10⅓ innings). His sinker velocity (90-93 m.p.h.) and changeup have shown improvement, and he’s added a cutter to diversify how he attacks hitters.

“The swings and misses are not there compared to previous years,” said Sox manager Alex Cora. “But I do believe just watching the last one, the last actually two or three, we’re comfortable that he can come up here and compete, throw a lot of strikes from a different angle and give us a chance to win.”

“Season’s been up and down for me, obviously, but I feel good. I think my best innings have been the most recent ones,” said Walter, citing command as the reason for his improvement. “I feel confident coming into this, and hopefully just help this team win.”

Cora said Walter likely will start Thursday, but left open the door to having him enter the game following an opener — presumably as a multi-innings pitcher. Either way, it will represent a hard-earned milestone that will bring a coterie that includes Walter’s parents, girlfriend, high school coach, agent, and offseason trainer to Target Field.

“It’s a dream come true,” said Walter. “Obviously everybody who plays baseball growing up wants to make it to this level and be able to compete and be able to contribute to a winning team. No better team to do it with than the Boston Red Sox, with so much history and I’m excited. I can’t even put it into words how excited I am to be here.”

Alex Speier can be reached at alex.speier@globe.com. Follow him on Twitter at @alexspeier.