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 visitors locker room at Madison Square Garden was 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 a 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 lost possession of the ball, and as Tatum lunged toward it, his right leg gave out and he crumpled to the court with 2:58 left. After Knicks forward OG Anunoby grabbed the loose ball and raced to the other end for a dunk, Tatum — who didn’t attempt to get up — 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 assisted off the court and into a wheelchair.

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

“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 a minimum the rest of this series.

Tatum had put together a fabulous game before the injury, scoring 42 points on 16-for-28 shooting, and had been a picture of durability over the course of his eight-year NBA career. He had never sat out a playoff game before a wrist injury cost him Game 2 against the Orlando Magic in the first round. The 10 games he sat out in the regular season were the most in his 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 many around the franchise in disbelief.

On the other side, the Knicks sent their well wishes to 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 looking 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.”