Harvey was diagnosed with thoracic outlet syndrome during an examination by Dr. Robert Thompson in St. Louis on Thursday, general manager Sandy Alderson said.
The issue involves a constriction of the opening in which nerves and blood vessels pass between the neck and shoulder. It is more of a nerve issue in Harvey's case, Alderson added.
Thompson presented Harvey with two options that are currently being weighed. Harvey could undergo surgery now, which requires a four-month period before returning to pitching. Or Harvey could attempt to delay the procedure by receiving a nerve-blocking injection.
Alderson suggested that the latter approach may not be effective and that surgery would ultimately be necessary anyway. Delaying the surgery could impact Harvey's availability in 2017.
"It's unclear how effective that would be or for how long," Alderson said about the nerve blocker. "... I do believe that surgery is probably inevitable and more a question of timing than anything else. So, obviously, to the extent that we're backed up for a period of time, it begins potentially to encroach on 2017 as well."
Alderson said the Mets were unaware Harvey was dealing with any shoulder discomfort until he complained after his last start. Harvey allowed six runs (five earned) on 11 hits in 3 2/3 innings against the Miami Marlins on Independence Day.
Manager Terry Collins said Harvey lost some feeling in the tips of his fingers.
"He came in the training room afterward and said, 'My shoulder is dead. My arm is dead. There's no energy there. I couldn't feel the ball,'" Collins said. "Obviously it was happening during the game. He didn't say anything until after the game."
Chris Young, Phil Hughes and Josh Beckett are other pitchers who have dealt with thoracic outlet syndrome.
Thompson likely would perform Harvey's surgery, according to Alderson. Harvey will return to New York from St. Louis and should make a decision in the next couple of days.
Alderson acknowledged there is no certainty Harvey will still be a frontline starter when he returns.
"Any time that you introduce a significant surgery of this type, I think you have to be cautious about what will happen," Alderson said. "But, at the same time, I fully expect that Matt will be back and ready to go in 2017."
Collins recalled managing John Hudek with the Houston Astros in the mid-1990s, when Hudek had surgery for thoracic outlet syndrome.
"He came back and pitched with it," Collins said. "I don't know if he was as effective as he was before he had it.
"Chris Young came back and had a good year in Seattle. So I think you can bounce back from it. But everybody is different, as we know. Anytime you have surgery, there's a level of concern."
The Mets' vaunted rotation is dealing with its share of injury issues. Steven Matz and Noah Syndergaard are pitching through bone spurs in their pitching elbows, while Zack Wheeler has endured a pair of setbacks in his return from Tommy John surgery and will not be back until late August at the earliest.
Right-hander Logan Verrett will start in place of Harvey on Saturday and should remain in the rotation after the All-Star break, Collins said. Matz's bone spur will need to be surgically removed, although the Mets are hoping anti-inflammatory medication will allow him to delay the procedure until after the season because it requires a three-month recovery time.
Wheeler received a cortisone injection last month for nerve irritation in his pitching elbow and has not yet returned to a mound. Alderson said he is "confident, not certain" that Wheeler will pitch for the Mets this season. Wheeler is planning to return to the slope of a mound this weekend for the first time since the June setback.
"He's had a couple of minor setbacks -- nothing serious, but serious enough that projecting any sort of return at this point is a little bit speculative," Alderson said.
In his second season back from Tommy John surgery, Harvey is 4-10 with a 4.86 ERA in 17 starts. Alderson said it is not a relief if those numbers were at least partly caused by a shoulder issue that Harvey did not disclose until after his last start. Alderson said Harvey would need to reveal how long the issue has been present.
"I'm not sure there's a sense of relief," Alderson said. "It's like using the word 'closure.' It's usually not really the case. In this case, given the potential implications with surgery as an option, it gives us some clarity in terms of where we go from here. Other than that, I'm not sure it's relief."
Alderson noted the minor league system is thin with potential rotation contributors, although he cited Gabriel Ynoa and Sean Gilmartin as potential considerations. Alderson added that the Mets will at least consider trading for a starting pitcher.
"We'll take a look at what's there," Alderson said. "We're obviously always comparing what's out there with what we have. So we'll just have to see."
Harvey's potential loss for the season comes at a terrible time. Captain David Wright recently underwent season-ending surgery to address a ruptured disk in his neck. And the Mets are set to open a four-game series against the first-place Washington Nationals on Thursday night. The Mets trail by four games in the National League East.
"I don't want it to sound like woe is me, but woe is us," Collins said. "I said something yesterday to one of the coaches. I said, 'Man, when do you think this is going to stop?' Can things possibly get worse? And the next day they continue to get worse. It's something you have to deal with. I know you can walk over on the other side of the field right now and they're not going to feel sorry for us."