The Celtics had a midnight deadline to land Kristaps Porzingis. In a stunning shift, they dealt Marcus Smart away to make it happen.

According to league sources, Boston will also send forwards Mike Muscala and Danilo Gallinari to Washington in the three-team trade. Boston will receive the Grizzlies’ first-round pick, No. 25 overall, in Thursday’s draft, as well as the Warriors’ first-round pick in the 2024 draft. Lastly, the Celtics traded the No. 35 overall pick in Thursday’s draft to Washington, and the Wizards received guard Tyus Jones from Memphis.

The addition of the 7-foot-3 Porzingis, a 27-year-old former All-Star who just completed the best season of his career, reshapes this franchise and gives coach Joe Mazzulla one of the league’s most unique offensive weapons.

But the reverberations from trading Smart, the 2021-22 NBA Defensive Player of the Year who has been the heartbeat of this team for years, will be significant. Smart, the sixth overall pick of the 2014 draft, was the first key piece of Boston’s swift rebuild following the Paul Pierce and Kevin Garnett era.

His intensity and toughness helped define the flawed but gritty Celtics teams early in Brad Stevens’s tenure as coach, and he morphed into a talented playmaker, often pointing out that after playing alongside former All-Star point guards Isaiah Thomas, Kyrie Irving, and Kemba Walker, he was the first to lead the Celtics to the NBA Finals, as he did in 2022.

He was active in the community, dyed his hair green, and seemed to fully embrace being a Celtic as much as any player in recent memory. But rosters and paths shift suddenly in the NBA, and now Smart is gone.

His departure clears the way for Derrick White, a breakout star of this season’s playoffs, to become the full-time starting point guard. White’s finest moment came in Game 6 of the Eastern Conference finals against the Heat, when his last-second putback improbably forced a Game 7 and kept alive Boston’s chances of becoming the first NBA team ever to overcome a 3-0 series deficit.

But the Celtics lost Game 7 at TD Garden, and that defeat may have set Wednesday’s chain of events in motion. Brogdon, meanwhile, appears slotted to return to his sixth man role after appearing to be on the way out. The Clippers’ recoil will raise new questions about Brogdon’s health, and whether he will ultimately need surgery on his injured forearm.

But the Celtics entered the offseason focused on reducing the congestion in their backcourt and adding to their frontcourt, and this trade has certainly done that.

Boston could have simply walked away when Los Angeles backed out of the three-team deal Wednesday, but Stevens, who is now president of basketball operations, attacked the situation with urgency, clearly viewing Porzingis as a key piece who could help the Celtics take the final step that has eluded them.

Porzingis rose to fame with the Knicks before an uneven stretch with the Mavericks. And this past season he bounced back with an excellent year in Washington, when he averaged 23.2 points, 8.4 rebounds, and 1.5 blocks while shooting 38.5 percent from the 3-point line. His arrival will allow Al Horford to slide back to his natural power forward position and provide Boston with yet another floor-spacing big man who can make life easier for All-Star wings Jayson Tatum and Jaylen Brown.

Porzingis would be eligible to sign a two-year, $77 million extension starting July 6, and the Celtics may have received assurances about his desire to be in Boston for the long-term before making this deal.

Gallinari’s Celtics tenure, meanwhile, comes to an end without him ever suiting up in green. The Italian forward tore his ACL playing in a World Cup qualifier last September and missed this entire season. Muscala was acquired from the Thunder last February to bolster the frontcourt depth but never cracked the regular rotation.

According to multiple league sources, the deal for Porzingis will likely signal the end of forward Grant Williams’s time in Boston. Last fall Williams was seeking a four-year extension worth approximately $54 million, but the two sides were unable to agree on terms.

Williams shot 39.5 percent from the 3-point line this season but fell out of Mazzulla’s rotation for much of the postseason. He is now a restricted free agent, giving Boston the ability to match any offer sheet he signs with another team.

But the addition of another frontcourt player combined with Boston’s ballooning luxury tax bill and tenuous position near the second apron in the NBA’s new collecting bargaining agreement would likely make the partnership unsustainable, sources said. The Celtics could still negotiate a sign-and-trade with Williams, allowing him to land with a team that does not have the salary-cap space to sign him outright, with Boston receiving assets in return.

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• Brad Stevens decided running it back wasn’t the move, so the Celtics struck first and acquired Kristaps Porzingis
• Your guide to the 2023 NBA Draft: After a seismic trade, Celtics back in first round on Thursday night
• The Celtics now hold the 25th pick in tonight’s NBA Draft. What have they done in the past at that spot?

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