As if the Knicks’ big-man crisis wasn’t serious enough, power forward Kenyon Martin sprained his left ankle in their 120-99 Atlantic-Division clinching win over the Wizards Tuesday night and had to be helped off the Garden floor. X-rays were negative, and no timetable was given for his return.

“He has a sprain, and he has to nurse it,” said coach Mike Woodson, who doesn’t expect the injury to keep Martin out of the playoffs but said he may well miss tomorrow’s game against the Bulls in Chicago. “We have to get some of our bigs back but we have time. We have five games left.’’

Martin — who started at center despite battling a sore left knee — came down on Chris Singleton’s foot with 10:28 remaining. Martin, who has a notoriously-high pain tolerance, banged his fist on the floor repeatedly as the crowd chanted his name. He was helped off the floor, putting no pressure on his ankle.

“It was tough [to see],” said Carmelo Anthony, who would rather see Martin sit until the playoffs if that what it takes to get to 100 percent. “I didn’t even want to go down there, because I know him. I know he gets injured and gets back up and shakes it off. Once I saw him pounding the ground I knew it was something. I’m just glad everything is OK, and his ankle will be fine. At this point we just have to get him right.

“I just tried to stress we need him now, but at this point we’d rather you just lock in and get healthy. ... If that’s what its going to take him to get back healthy and be full strength for playoffs, I’d rather not see him, and keep playing the five.’’