Mejia, 20, was forced to leave Wednesday's game against the Pittsburgh Pirates after flinching during a third-inning pitch that he bounced to Andrew McCutchen. A subsequent MRI exam at the Hospital for Special Surgery in Manhattan revealed a rhomboid muscle strain, the team announced.
Mejia began the season in the Mets' bullpen before being dispatched to Double-A Binghamton in June to resume working as a starter and hone his curveball and changeup. He had returned to the Mets Wednesday and was making his third start.
Mejia, the organization's top pitching prospect, should be a contender for a rotation spot next spring training. Mejia went 0-4 with a 4.62 ERA in 33 appearances (three starts) in 2010.
Mejia's next scheduled turn falls on a team off-day on Monday, so the Mets could elect to go with four starters in the short term. Beyond that, left-handers Pat Misch, Raul Valdes and Oliver Perez would be candidates to fill that spot during the season's final two weeks.