Stewart (hamstring) will undergo an MRI on Wednesday, Stephen J. Nesbitt of the Pittsburgh Post-Gazette reports.
While the injury originally wasn't thought to be anything overly serious, Stewart tried testing it out prior to Tuesday's contest and did not respond well, prompting a trip to the DL. A possible timetable for his return should become clearer following Wednesday's tests. In the meantime, Elias Diaz will serve as the Pirates' backup catcher.