BOSTON (AP) — When Jayson Tatum toppled to the court, writhing in pain and grabbing at his right foot late in the Celtics’ Eastern Conference semifinals Game 4 loss to the New York Knicks, it looked like a potentially season-altering injury.

Boston’s worst fears are now confirmed.

Tatum had surgery Tuesday, May 13, to repair a ruptured right Achilles tendon that will sideline him for the remainder of the playoffs, the team announced.

The Celtics provided details on the extent of Tatum’s injury and announced news of the surgery a day after the six-time All-Star went down in the Celtics’ 121-113 Game 4 loss to the New York Knicks. It puts the hopes of the defending champions repeating and Tatum’s playing status for next season in doubt.

They did not give a timetable for his return, but said a full recovery was expected.

When Kevin Durant tore his Achilles tendon during the 2019 NBA Finals, he wound up missing the entire 2019-2020 season.

The 27-year-old Tatum is leading the Celtics in points (28.1), rebounds (11.5) and assists (5.4) per game for the second straight postseason.

The Knicks lead the Celtics 3-1 in their Eastern Conference semifinals series. Game 5 is in Boston on Wednesday night.

Tatum was carried off the court with 2:58 remaining in Monday night’s game. The Celtics had just turned the ball over and as Tatum moved for the loose ball, his leg gave out and he went down. He buried his face in a towel in obvious pain while grabbing at his leg above the ankle after the noncontact injury.

Tatum scored 42 points, his highest total during these playoffs and one of the best all-round postseason performances of his career, before he was hurt.

The injury was Tatum’s second this postseason. He missed Game 2 of Boston’s 4-1 first-round series win over Orlando with a bone bruise in his right wrist. It was the first time he’d missed a playoff game in his career.

Now, Tatum’s teammates will face a daunting task that few teams before them have pulled off if they want to break an NBA record six-season drought without a repeat champion.

Teams holding a 3-1 lead in the NBA playoffs have gone on to win 95.6% of the time, with only 13 teams in 293 tries ever coming back from the deficit to win the series.

“Obviously, we all realize in our heads what this could mean. This part of the sport – it’s tough,” Celtics center Kristaps Porzingis said Monday. “We have to move forward. He don’t want us to be over here sad and not play our best basketball.”

To become the 14th team to overcome a 3-1 deficit, the Celtics will need big performances from a roster that has several players who have dealt with injuries this postseason.

All-Star Jaylen Brown entered the playoffs dealing with a right knee issue that has slowed him at times. Porzingis also has been slowed at times by the lingering effects of a viral illness that sidelined him for stretches in the second half of the regular season.

Brown said Monday that his confidence remains high.

“Get ready for the next one. Get ready to fight. Get ready to come out on our home floor and do what we need to do,” Brown said. “That’s the goal. Still the goal. We’ve got enough in this locker room. So, I believe in my guys.”

Regardless of how this series or the rest of the playoffs play out for Boston, Tatum’s injury likely will alter how the franchise confronts this coming offseason.

Including this year, the Celtics' payroll will be over the salary cap and make them a luxury-tax team for the third straight season. That means they will be hit with the “repeater tax” for being over the cap threshold in three out of four seasons.

Their current payroll for next season is on track to come in around $225 million, which would mean a tax bill next year of almost $280 million. The combined potential $500 total price tag would be a league record.

It is unclear whether the team’s incoming new ownership will want to keep paying those hefty taxes to maintain the current roster after agreeing to a purchase in March that is expected to have a final price of a minimum of $6.1 billion.

Tatum signed an NBA-record five-year, $314 contract last July that will begin next season. Brown is playing under a five-year, $304 million deal that kicked in this season.

That means that trimming player salary is a possibility ownership could explore this summer.

And with Tatum possibly missing all of next season, it could accelerate the front office’s timeline for reworking the current roster with an eye toward the future.