Jayson Tatum underwent surgery Tuesday to repair a ruptured right Achilles tendon, ending his season and putting his availability for the 2025-26 campaign in jeopardy.

“No timetable is currently available for his return,” the Celtics said in a statement confirming the diagnosis, “but he is expected to make a full recovery.”

Tatum, the NBA’s highest-paid player and one of its most durable superstars, suffered the non-contact injury with three minutes remaining in Monday night’s Game 4 loss to the New York Knicks at Madison Square Garden.

While attempting to beat Knicks wing OG Anunoby to a loose ball, he fell to the floor, clutched at his right ankle and had to be carried off the court by Celtics staffers. President of basketball operations Brad Stevens stepped in to help guide his star player, who was unable to put weight on his injured leg, down the tunnel.

ESPN’s game broadcast then showed Tatum, face buried in his hands, being transported by wheelchair toward the visitors’ locker room.

Trailing by seven at the time, the Celtics went on to lose 121-113 to fall behind 3-1 in the best-of-seven Eastern Conference semifinals.

“Obviously, you’re always worried about someone’s health,” Celtics head coach Joe Mazzulla said. “So the fact that he had to be carried off — he’s the type of guy that he gets right up. He didn’t. And we’ll know (Tuesday) exactly what it is, but yeah. I mean, it’s tough to watch a guy like him get carried off like that.”

Tatum’s injury left his teammates shell-shocked.

“I think tonight is tough,” a visibly emotional Jaylen Brown said at the postgame podium. “I think everybody’s kind of at a loss for words, just because, one, losing the game, but obviously the concern with JT. But we pick our heads back up tomorrow and go from there.”

“I mean, at this point, I’m concerned about Jayson,” Al Horford added. “That’s the most important to me. The game stuff, we’ll address it. But I’m just hoping that he’s OK.”

Since entering the NBA as the third overall pick in the 2017 draft, Tatum leads the league in games and minutes played, including playoffs, and ranks second in total points behind Milwaukee’s Giannis Antetokounmpo. He’s never missed more than 10 games in a season and didn’t sit out his first postseason game until last month, when a bone bruise in his wrist sidelined him for Game 2 of Boston’s opening-round series against Orlando.

Tatum is a six-time All-Star, and he almost certainly will be named to his fourth consecutive All-NBA first team when it’s announced later this month. Last season, after a half-decade of near-misses, he helped propel the Celtics to their elusive 18th championship, leading the team in points, rebounds and assists during both the playoffs and the NBA Finals.

To extend their season and keep their bid for back-to-back titles alive, the Celtics now must win three consecutive games without their best player — and with a depleted supporting cast. Injuries hit Boston’s roster much harder this season than they did during last year’s title run, and several core rotation players have either missed time or been limited during the first two playoff rounds.

That list includes Brown, who’s battled a troublesome knee injury since before the NBA All-Star break in February, and starting center Kristaps Porzingis, who’s been slowed by symptoms from a lingering viral illness that have zapped his energy and impacted his availability. Both players struggled in Monday’s loss to the Knicks, during which the Celtics blew a double-digit second-half lead for the third time in the series. Starting guard Jrue Holiday (hamstring) and reserve wing Sam Hauser (ankle) each have missed three games this postseason.

Tatum was terrific before his injury, recording 42 points, eight rebounds, four assists, four steals and two blocks in one of the best all-around playoff performances of his NBA career. Game 5 is Wednesday night at TD Garden.

“It’s tough to see him go down, but we’ve got to find a way to win Game 5,” said Derrick White, the only Celtics starter who’s been fully healthy this postseason. “That’s where we’re at right now. We’ve just got to find a way to win.”

The ramifications of Tatum’s loss will be felt beyond this season, as well. Though the Celtics did not announce a projected timetable for his return, Achilles injuries often require recovery periods of a year or more. Kevin Durant missed 18 months after he ruptured his Achilles during the 2019 NBA Finals. Klay Thompson was out for 14 months after he did the same in 2020. Both were 30 at the time of their injuries, three years older than Tatum, who is squarely in his prime at age 27.

With the 2025-26 campaign set to tip off in less than six months, Tatum is virtually guaranteed to miss at least part of next season, if not all of it. That could have major consequences for the rest of Boston’s roster, which already was likely to undergo difficult changes this summer. As currently constructed, the 2025-26 Celtics are on track to be the most expensive team in NBA history, costing close to $500 million between player salaries and onerous luxury tax penalties.

Tatum is set to earn $51.4 million next season as the record-setting supermax extension he signed last July kicks in. Though the NBA does offer some relief for teams in this situation in the form of the disabled player exception, that would only allow the Celtics to sign a player who makes less than the mid-level exception, which is projected to be around $14.1 million — nowhere close to Tatum’s salary. That clause also only applies to players who are expected to miss the entire season.

Players like Holiday, Porzingis and Hauser already were potential offseason trade candidates, regardless of how this playoff run ended. Time will tell whether Tatum’s forthcoming absence — which seemingly slams shut Boston’s championship window in the short term — will trigger an even more radical roster overhaul.