"Jeff was obviously very aggressive in his bullpen, because he was going in to pitch the 15th inning," Pirates manager Clint Hurdle said. "He's been a very honest self-evaluator for us."
Locke will start on Wednesday instead, and Gerrit Cole will still start Thursday.
Cumpton made two starts in mid-June and allowed four runs, two walks and 12 hits over 10 innings. He struck out eight in two no-decisions.
"Cumpton was probably 13th on our depth chart coming out of Opening Day," Hurdle said. "And for him to come up and pitch two efficient ballgames, giving us a chance to win two Major League games ... he wasn't out of place. His pace and rhythm were good."
The club optioned infielder Josh Harrison to Indianapolis to make room on the 25-man roster.
Steven Petrella is an associate for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.