NEW YORK — The reigning champion Boston Celtics find themselves one game away from playoff elimination after a 121-113 loss to the Knicks in Game 4 of the Eastern Conference semifinals, a result that put New York on the cusp of its first conference finals appearance in 25 years.

But the 3-1 series deficit isn’t why the visiting locker room at Madison Square Garden was completely silent for a half-hour after Monday’s game. It was because of the potentially devastating news awaiting Celtics superstar Jayson Tatum, who had to be carried off the court late in the fourth quarter after suffering an apparent noncontact lower leg injury.

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

The Celtics had just turned the ball over, and as Tatum lunged toward the loose ball, his right leg gave out and he crumpled to the ground in a heap with 2:58 left in the game. After Knicks forward OG Anunoby raced to the other end and dunked the ball, Tatum — who didn’t even attempt to get off the ground — signaled for a timeout and put a towel over his face in obvious pain while grabbing at his leg above the ankle.

He eventually was helped to his feet and carried off the court, and then was put in a wheelchair to take him into the back halls of the arena.

Celtics coach Joe Mazzulla said Tatum would undergo an MRI on Tuesday, but the team didn’t put out any official statement on Tatum’s injury, and Mazzulla had no other update on his condition.

“I’m not sure,” Jaylen Brown said when asked what Tatum’s injury could mean for the franchise moving forward. “I’ve got no words right now.”

For the Celtics, who entered Monday’s game as the odds-on favorite to emerge from the East despite being down 2-1 after leading the first three games of the series by at least 20 points, the challenge will be finding a way to regroup after losing their best player and leader for what all but certainly will be — at an absolute minimum — the remainder of this series.

Tatum had put together a fabulous game before the injury, going for 42 points on 16-for-28 shooting, and had been a picture of durability over the course of his career. He had never missed a playoff game before a wrist injury cost him Game 2 against the Orlando Magic in the first round. The 10 games he missed in the regular season were the most he’d ever missed in his NBA career.

Sitting at his locker, Celtics center Kristaps Porzingis said he and his teammates have no choice but to push forward and try to make up for Tatum’s absence as they attempt to advance to the Eastern Conference finals for a fourth consecutive season.

“These things happen,” Porzingis said. “Obviously we all felt for him in that moment, but we just have to keep going, we have to keep playing. Obviously we all realized in our heads what this could mean, but again, this is part of the sport, it’s tough, and it’s hard to see and hard to accept the truth.

“But, it is what it is and we have to go forward with what we have now.”

The double whammy of suffering a third loss of a double-digit lead — Boston led by 14 points before New York stormed back in the second half — and Tatum’s injury left everyone around the franchise in a state of disbelief.

On the other side, the Knicks sent along their well wishes for Tatum, with Karl-Anthony Towns — who had a serious calf tear a couple years ago that initially looked like a torn Achilles — saying he said a prayer for the All-Star forward to be back on the court as soon as possible.

“I’ve been in a situation like that with my calf [before],” Towns said, “so I just walked up, respectful of his space, and just prayed to my mother, prayed to God to put protection over him, comfort. Whatever he needs to go through, I hope it’s a quick, painless process. I’m hoping we can see him back here soon, with the superstar talent he is, playing at the highest level.”

The Celtics will be hoping for the same thing, while also hoping to keep their season alive in Game 5 on Wednesday night in Boston.

“Well, it’s very concerning,” Horford said. “Just from the care that I have for him and what he means to us, what he means to Boston. Just very tough for us.

“More importantly, it’s just really tough for him right now. Let’s see what happens. Just is very tough for our group altogether.”