The Pirates announced that Charlie Morton experienced "increased discomfort" in the latter innings of his last start and again during his bullpen session. He is scheduled to have a follow-up exam next week and then a decision will be made how to proceed.
Vance Worley will start Sunday in Morton's absence.
Morton was unsure if this setback signaled the end to his 2015 season.
"[The pain] is very different," Morton said. "The degree and the intensity of the discomfort is different than it has been in three months."
Clint Hurdle said no final decision will be made on Morton until after his reevaluation.
Hurdle was not concerned about Worley showing any rust after being skipped in the rotation.
"I think more often that not it is a great idea to have a guy skip [a start] sometime during the season," Hurdle said. "To skip a start, for me, is smart thing to do from time to time ... [Worley's] had a very solid major league workload since he has been up, compared to last year. I think this was a good opportunity for him to catch his breath."