The right-hander started three games earlier this season and worked four innings in relief of Charlie Morton on Thursday in Cincinnati.
With top prospect Brad Lincoln ruled out by manager John Russell as a potential replacement for Morton, the Pirates' only likely option other than plucking Karstens from the bullpen would be to promote Daniel McCutchen from Triple-A Indianapolis.
But McCutchen was ineffective while up with the Pirates earlier this season, going 0-2 with a 13.50 ERA in four games.
Karstens pitched six scoreless innings in a 2-0 victory over the Cardinals on May 8, allowing three hits while walking two and striking out four. He has made six relief appearances since, the longest when he followed Morton in Cincinnati.
The 27-year-old former Yankees pitcher gave up four hits and a run to the Reds after Morton had been battered for seven runs in the first two innings. Morton was put on the disabled list the next day with shoulder fatigue, opening a spot in the rotation.
Karstens was 1-1 with a 4.08 ERA in his three starts and is 0-0 with a 5.65 ERA in relief. His overall ERA is 4.78 and he has 19 strikeouts and 10 walks in 32 innings, allowing 32 hits.
After starting 13 games and making 26 relief appearances with Pirates in 2009, Karstens began this season in the Indianapolis bullpen. He made one start for the Indians before being called up.