Colin Dunlap of the Post-Gazette is reporting on Twitter that Jeff Karstens will get the start on Thursday in place of the injured Ross Ohlendorf. Karstens really seems to be the only logical candidate at the moment with Brian Burres and Rudy Owens not on the 40-man roster, Scott Olsen injured and Brad Lincoln having only made one rehab start in AAA.
I think if Ohlendorf is out more than a few starts, Lincoln will eventually take the rotation spot and I wouldn't even be surprised if he got the second start if he is effective in his next outing. I know the Pirates would like to keep Karstens in the role of long-man.
UPDATE: With the rainout, it is now expected that Karstens' start will be pushed back to sometime over the weekend against Cincinnati.