The season-ending injury is a catastrophic blow to one of the NBA's young faces and the Celtics, who now face an even stiffer challenge to overcome the Knicks 3-1 series lead.
Celtics are used to seeing Tatum get up when he goes down, like he did after a hard fall that caused a wrist injury in the first round of the playoffs.
When he couldn't get to his feet late in Game 4 and had to be carried off the court, it was clear just how serious his lower right leg injury was, even before the testing.
"He's the type of guy, he gets right up," Celtics coach Joe Mazzulla said. "So he didn't and we'll know tomorrow exactly what it is. But yeah, I mean it's tough to watch a guy like him get carried like that."
The Celtics had just turned the ball over with about three minutes left and as Tatum lunged forward toward 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 non-contact injury.
The New York Knicks won 121-113 to take a 3-1 lead. The series resumes Wednesday in Boston with the Celtics needing to win to extend their NBA title reign.
The team says there is currently no timetable for Tatum's return, but it is expected to be a long rehab process. He is expected to make a full recovery.
The Associated Press contributed to this report.
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