Wallace said Yao Ming kicked him as the two were running down the court in the second quarter. The 6-foot-9 Wallace managed to play midway into the third quarter before leaving for good with 6:19 left.
"I thought it was like a calf bruise or something, that would probably let up eventually," Wallace said. "But it didn't. It was one of those freak accidents that happen in basketball."
Wallace saw the X-rays taken at the Toyota Center that showed the broken fibula.
"I know what that little white line means," he said.
Wallace, who guarded Yao for most of his 17 minutes, was playing his fourth game since missing Cleveland's Feb. 18 win over Toronto with a right arm laceration that required 14 stitches.
Wallace was averaging 6.6 rebounds and 3.0 points in 53 games this season. His loss could be devastating and continues a run of bad injury luck for Cavs, who have played without Zydrunas Ilgauskas (left ankle) and Delonte West (broken wrist) for extended periods.
"It's a tough blow for us," said Cleveland coach Mike Brown.
Wallace has been Cleveland's best interior defender and has played well in his second season with Cleveland, which acquired him in a trade from Chicago last February.
The Cavaliers passed up chances to acquire a big man before the trade deadline. Now, they'll have to rely on Ilgauskas, Anderson Varejao and maybe even 6-9 rookie J.J. Hickson to handle opposing centers inside.
"We just have to have somebody step up and play some quality minutes for us," Brown said.
Wallace had a protective boot and crutches next to his locker after the game. He was hoping to play again before the postseason.
"I haven't had a broken arm, broken leg, anything major like that since I've been playing basketball," said Wallace, in his 12th season. "For it to happen now, it's just tough. I'm 34 years old. I don't need no broken bones."