Leiter injured his left knee while racing to cover first base in the seventh inning on a slow roller hit to the right side by Derek Jeter. Leiter hurt the knee while trying to tag out Jeter.
Leiter tried to continue, but pulled himself after making one warmup toss. Mel Rojas came in and gave up a three-run homer on his first pitch to Paul O'Neill that sent the Yankees to an 8-4 win.
An MRI Saturday morning revealed that Leiter had suffered a 50 percent tear of his patella tendon. Leiter's knee will be immobilized by a brace for the next few weeks and he'll begin extensive therapy. He is expected to miss four to six weeks.
"Obviously, I'm very disappointed," said Leiter, who stayed away from Shea Stadium to rest on Saturday. "Even though the doctor said four to six weeks, I guess hopefully I'm not going to need that and hope that resting it that it's going to be quicker than that."
If Leiter's knee doesn't respond after a few weeks, he would likely undergo season-ending knee surgery.
Leiter, acquired in a winter trade with Florida, has been the Mets' most consistent starter all season. Even with a loss on Friday, he's 9-4 with a 1.86 ERA. He would have been a lock to make his second All-Star team.
"It's such a downer, such a bummer," he said.
The news of Leiter's injury was obviously a major disappointment for the Mets, who through trades have positioned themselves for a playoff run this season.
"That's a big shot right there," said closer John Franco. "It's going to be a tremendous loss for us. Everybody's going to have to step and whoever it is (in Leiter's rotation spot) it's going to be big shoes to fill."
Mets manager Bobby Valentine said he would speak with his coaching staff before deciding who might replace Leiter in the rotation.
Among his short-term options, are lefties Brian Bohanon (who has started three games this year), Bill Pulsipher or possibly a minor league recall. Mark Mimbs, a left-hander, won his ninth game for Triple-A Norfolk on Friday.
Armando Reynoso, on the 60-day disabled list with a strained right elbow, is making rehab starts in the minors and could be a long-term answer if Leiter is out for the season.
Mets general manager Steve Phillips said the club would also look outside the organization for pitching help.
To immediately fill the roster spot, the Mets purchased the contract of reliever Jeff Tam from Norfolk.
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