Mets general manager Sandy Alderson confirmed Friday that lefthander Steven Matz will be out three weeks and possibly even longer with a partial tear of his left lat muscle.

"No one has guaranteed to us that it will be healed in three weeks," Alderson said during a news conference at Citi Field. "These things are very difficult to project. Based on what the doctors have told us, there's a very high likelihood this will heal without a problem."

Alderson said the former Ward Melville star noticed the discomfort after his first start against Cincinnati on June 28 and subsequently was monitored by doctors, who determined that he could pitch in Los Angeles last Sunday.

Alderson said he made the decision to allow Matz to pitch against the Dodgers. "There's no way to knowing whether that start or his start against Cincinnati aggravated this injury, precipitated it or anything of that sort," he said.

After the start against the Dodgers, in which Matz allowed two hits in six shutout innings, Alderson said it was decided that Matz would skip his next start, scheduled for Sunday against Arizona at Citi Field, and that Jonathon Niese would pitch.

Matz played catch Tuesday, tossing about 10 times, and was shut down, Alderson said. An examination Thursday revealed the partial tear.

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Matz will not throw for three weeks. He will be examined at that point and, if the healing has progressed to the point that the doctors are satisfied, he will "pass on to a different phase," Alderson said.

"If the healing hasn't gotten to a point where we're satisfied, we'll probably be in a similar situation," he said. "It's all a function of how his muscle responds to the next three weeks."

"It's definitely frustrating, but it could be worse," said Matz, who had Tommy John surgery in 2010. "It's a really minor thing. I'll get through it and be back. I'll try and stay positive."

Matz added that doctors told him that if this had occurred in the World Series, they wouldn't "jump in front of the door and stop me."

"It was just the risk of possibly tearing it completely," said Matz, who is 2-0 with a 1.32 ERA since his promotion from Triple-A Las Vegas.

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Alderson doesn't agree that it's a minor injury but said it's not the worst injury that could have happened.

"The act of pitching is not a natural one," he said. "These things come up from time to time . . . If we got an MRI on every pitcher who's ever had mild pain, we'd probably be getting them daily, or on a somewhat frequent basis."

Matz said he never had experienced a lat injury.

"It was kind of weird because it was something that I had never really felt before," he said. "It wasn't killing me, but something felt different."

Alderson offered a possible explanation for this, saying that doctors told him that Matz's left lat muscle is smaller than his right one.

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"It's possible that this injury existed before and was just asymptomatic, based on the size of the muscle," he said. "We don't know when it happened. But what we're most concerned about now is not aggravating the injury. These partial tears can heal on their own. They do in 90 percent of the cases."