Karstens was initially supposed to pitch on Thursday, following Kevin Correia and Paul Maholm in a three-game series against Milwaukee. But with no game played on Tuesday, Karstens is back and available in the bullpen for the remaining pair of games against the Brewers.
Charlie Morton will take the mound Friday in Cincinnati, with James McDonald now scheduled to start on Saturday. The Pirates will need another starter on Sunday, but manager Clint Hurdle is leaving the letters "TBD" in that spot, for now.
The start will likely go to Karstens, who was initially pegged to take Ross Ohlendorf's spot in the rotation while Ohlendorf is on the disabled list. Karsten's availability would change, however, should he be needed in long relief on either Wednesday or Thursday.
Another option for the Pirates on Sunday would be Brian Burres, who is scheduled to pitch for Triple-A Indianapolis that day. Working against Burres, though, is the fact that he would have to be added to both the 25- and 40-man rosters before being called up.