There is no timetable for Lagares' return, but intercostal strains often take two to four weeks to heal.
"I don't think you can ever put a date on these kinds of things, these intercostal strains or those oblique tears or anything else," manager Terry Collins said. "That's already what I told him yesterday morning when he said he was OK. I said, 'You've got to be honest, because if it gets worse, it's not two weeks. It could be four.' So we'll have to wait and see."
Lagares' injury comes at a time when he had finally begun earning everyday playing time in center field. In 42 games, Lagares is hitting .288 with two homers, 18 RBIs and 19 runs scored.
In his absence, den Dekker and Chris Young will split time in center field. The Mets are currently playing with a four-man bench in an effort to assuage their taxed bullpen, and should call up another outfielder -- either Kirk Nieuwenhuis or Andrew Brown, both of whom are capable of manning center -- at some point this week.
"He's going to get some playing time, and we'll find out what we've got," Collins said of den Dekker. "Before he broke his wrist last spring, he was the guy. He was the guy. I had Minor League guys telling me at one time that he and Juan were neck and neck in center field. We've seen what Matt can do defensively, so certainly we're not afraid to put him out there."
Eric Young Jr., who cut into Lagares' playing time earlier this season, remains sidelined with a strained right hamstring and is not eligible to return until June 10. Young will begin an outdoor running program Tuesday in Port St. Lucie, Fla.
Until either he or Lagares returns, the Mets will have to mix and match in their outfield. Collins does not blame Lagares for playing in Sunday's game despite his injury, believing he was fully healed when he took the field.
"We did our due diligence," Collins said. "They tested him in the training room. They put him through some things where they said if it had been acting up, they'd have seen it. He said there was absolutely nothing wrong, and then we took him downstairs to hit. Obviously, there were four sets of eyes on him to see if he was flinching or if his swing changed, and he came out of there feeling great. Then all of a sudden as the game went on, he just started getting worse."