The series against the Phillies is four games, which means that everyone who's currently in the rotation is scheduled to start, except Kevin Correia. That suggests that Correia is probably safe, although it's also possible that Karstens could start Thursday, on Brad Lincoln's turn, and Lincoln could pitch on Friday. Correia starts today, of course, and I'm sure he could force the Pirates' hand if he pitched really badly.
Getting Karstens back should provide a nice boost.
UPDATE: Tom Singer says there's some possibility that Karstens could start today, in place of Correia. Stay tuned.