Charlie Morton was called up from Triple-A Indianapolis to start against his former team, but the Pirates right-hander lasted only one inning before a tight right hamstring ended his night.
Pittsburgh turned to Karstens (3-4), who was bumped to the bullpen by Morton's promotion. The former starter gave up one run in 4 2-3 innings to earn the win, snapping Pittsburgh's three-game losing streak.
Matt Capps pitched the ninth for his 14th save, giving up a two-out homer to Garret Anderson. Jair Jurrjens (5-4) took the loss.