Diaz was removed from Saturday's game against the Braves with right trap tightness, Anthony DiComo of MLB.com reports.
Diaz entered for the ninth inning with the Mets trailing 7-5, and he allowed two runs on one hit and one walk before exiting with the athletic trainer. Manager Mickey Callaway said "it's really hard to say" whether the injury could land Diaz on the injured list, but he at least seems unlikely to be available for Sunday's series finale.