Papelbon would provide good depth, but I have a feeling there will be an equally good reliever available who would cost less in prospects.
Assuming Buchholz is able to pitch in the second half, I'm still not sold on the idea that we need a starter. I can't believe I'm going to say this, but Buchholz and Lackey are about as good a top two starters as you're going to find on any team this year. Lester could be a quality #3 if he fixes his mechanical issues, if not Dempster would make for an average #3. Maybe we get lightning in a bottle with Doubront, De La Rosa, Webster, Ranaudo or some other young pitcher. If not, we should still have the depth to make the playoffs. There aren't going to be many quality starting pitchers available, and those that are available and going to come at a steep price.