Harvey's issue is not considered serious and may be resolved within a couple of days, a source said Tuesday.
General manager Sandy Alderson on Monday declined to specify the issue, other than to say it was not related to an elbow or shoulder or anything else orthopedic. Harvey reported the issue to Mets personnel early Monday after a symptom unexpectedly arose, Alderson added.
"It's a non-baseball medical issue that we have to address," Alderson said after Monday's 3-1 loss to the St. Louis Cardinals. "It came up this morning as far as I know. There will be some follow-up tests and consultation that will take place over the next couple of days."
Asked if there is any gravity to the undisclosed issue, Alderson added: "I think it's a little early to attach any level of concern. ... I think we need to wait for more medical information before we decide it's of concern, or great concern, or no concern."
Harvey will be scratched from Tuesday's Grapefruit League start against the Miami Marlins. That had been slated to be his final tuneup for Opening Day opposite Kansas City Royals right-hander Edinson Volquez at Kauffman Stadium in a rematch of Game Nos. 1 and 5 of the World Series.
Harvey may be dispatched to New York for additional tests, Alderson said.
"It's way too premature for us to be discussing anything related to Opening Day," Alderson said. "I understand Opening Day is not too far away, but we're dealing with tomorrow, and we should know something more tomorrow -- or the next day. But right now he's not pitching tomorrow. That's kind of where the story ends."
Said manager Terry Collins: "We're very lucky to have the pitching we've got in case he can't go that we'll have somebody else ready."
Agent Scott Boras could not be reached for comment.
Alderson cautioned against minds running wild.
"If he had to have a wisdom tooth pulled, he wouldn't pitch tomorrow," Alderson said. "So let's not jump to conclusions here."