Matz felt a twinge near his left armpit after throwing 110 pitches during a dazzling major league debut June 28 against Cincinnati. But he was cleared to start again Sunday in Los Angeles, where he tossed six innings of two-hit ball in an 8-0 victory over the Dodgers.
Despite the excellent results, Matz was diagnosed Thursday with a partial tear of the lat muscle on his left side. He received a platelet-rich plasma injection and won't throw for three weeks before getting examined again.
That means Matz will be sidelined at least a month - probably longer.
''It's definitely frustrating, but it could be worse,'' Matz said before the Mets hosted Arizona. ''I'll get through it and be back. So I'm just trying to stay positive.''
Well aware Matz might not be 100 percent, New York general manager Sandy Alderson said he made the decision to allow the 24-year-old lefty to start in Los Angeles after Mets trainers consulted with team doctors.
''There was lots of conversation about it and consultation with Steven as well as the doctors, and at the time we felt that he was capable of making the start. There's no way of knowing whether that start or his start against Cincinnati aggravated this injury or precipitated it or anything of that sort,'' Alderson said.
''We don't know when it happened. But at the same time, 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, and so while I wouldn't necessarily agree with Steven that this is a minor thing, at the same time it doesn't rise to the level of some of the other injuries that a pitcher can sustain,'' he said.
Matz has won both his big league starts, compiling a 1.32 ERA and 14 strikeouts in 13 2-3 innings. He said the injury has never really caused him pain, and he didn't feel it on a particular pitch.
Alderson suggested it's possible the injury existed before and ''was just asymptomatic.'' Especially because Thursday's exam revealed the lat muscle on Matz's left side to be smaller than the one on his right.
''It's just like a little pulling. It's hard to describe,'' Matz said. ''It didn't hold me back at all and I felt good enough to go out there and pitch.''
Alderson said even before the MRI, the Mets decided to skip Matz's turn Sunday against the Diamondbacks. He indicated left-hander Jonathon Niese had already been informed he would start that day, on full rest.
''Just another bump in the road that we've got to get through and have somebody else step up,'' manager Terry Collins said. ''We did all the things we thought were right.''
Now the Mets are discussing whether to stick with a six-man rotation after next week's All-Star break or go back to the traditional five-man cycle. Matz's promotion from the minors late last month was part of the team's plan to return to a six-man rotation while managing innings for their prized young starters, including Matt Harvey, Noah Syndergaard and All-Star Jacob deGrom.
''We have the option of going back to five with perhaps spot starters. I think it's going to be a function of what we see as our options for that sixth starter, and we won't really have to address that until probably three to five days after the break,'' Alderson said.
''Fundamentally, we're left with two choices: Either we pitch five guys - innings limits be damned - until the end of August and we see where we are, or deal with it as we had approached it before, which is to say we want to preserve the ability of our pitchers to go beyond September,'' the GM explained. ''We'll resolve that in the next few days.''
Matz was placed on the 15-day disabled list, retroactive to July 6, and the Mets recalled infielder Danny Muno from Las Vegas. Muno has been back and forth from the minors several times and was 2 for 24 (.083) in 13 games with New York this season.
In other injury news, Alderson said lefty reliever Jerry Blevins (broken left forearm) received a second opinion in Boston and doctors determined his arm has not healed well enough yet for him to begin throwing.
''He will be re-examined in the proverbial three weeks,'' Alderson said.
Slumping left fielder Michael Cuddyer homered in the first inning after making only two starts in the previous eight games because of a sore left knee. Cuddyer was examined Thursday and doctors told him if he can tolerate the discomfort, he can play, Alderson said.
Cuddyer had an MRI on July 1 that revealed a slight strain and was given two shots (cortisone and a lubricant) to relieve the pain.
''He'll probably have it all year,'' Collins said. ''Now that he knows it, he said he's OK to play.''