"We're still trying to get him completely healthy," said Pirates manager John Russell. "We're not in any rush."
Karstens, who's been sidelined twice this season with discomfort in his arm, is 3-10 with a 4.88 ERA in 25 appearances, including 19 starts.
While Karstens is eager to return to the mound, Russell is reluctant to expedite his recovery.
"For him, I know he wants to pitch," Russell said. "He wants to go out on a solid note. But we're not going to push the issue."