He first suffered the injury making a catch over the weekend and went back to New York for further tests Monday, per Tim Healey of Newsday.
General manager Brodie Van Wagenen said Tuesday that it's "too early to know" whether or not he will be available for Opening Day. The Mets are scheduled to face the Washington Nationals in the first game of the season on March 26.
An extended absence for Conforto could be damaging for the Mets after the left-hander hit 33 home runs last season, second most on the team behind only MLB leader Pete Alonso. He also added a .257 average with an .856 OPS and 92 RBI.
It was the third straight year with at least 25 home runs for the one-time All-Star.
His absence could leave the Mets shorthanded in the outfield, especially with Yoenis Cespedes still working his way back from an ankle injury. Though he intends to be ready by Opening Day, per Anthony DiComo of MLB.com, he hasn't played a regular-season game since July 2018.
It leaves the team with only two healthy natural outfielders: Brandon Nimmo and Jake Marisnick.
Jeff McNeil, J.D. Davis and Dominic Smith also spent time manning corner outfield spots in 2019 and could handle increased roles again if needed, although McNeil is expected to play more third base this season.