"It feels a little bit tight," Mejia said.
Noting that the injury first began vexing him earlier this week, Mejia said he received an in-game massage that reduced his discomfort. But he began feeling it again during his 18-pitch 12th inning on Thursday, forcing him to retreat to the clubhouse after recording the third out. Carlos Torres came on and ultimately gave up Bryce Harper's walk-off homer in the 13th.
Mejia also pitched through a sore back last weekend in New York, and he has a long history of arm issues dating back to 2010. The Mets believe his latest ailment falls toward the less-serious end of the spectrum.
"We'll find out more [Friday]," manager Terry Collins said. "Right now, the trainers don't think it's anything serious. But it was serious enough that he wasn't going to pitch anymore today."
If Mejia is unable to pitch this weekend, Jeurys Familia and Vic Black would be the leading candidates to replace him at closer.
Anthony DiComo is a reporter for MLB.com. Follow him on Twitter @AnthonyDicomo. This story was not subject to the approval of Major League Baseball or its clubs.