The Tigers did not give up a ton for Fister. They gave up a lefty with no stamina, a role player, a MiLB 3B who is raw and can't defend, and a PTBNL.
I understand that it's hard to get value at the deadline, but there are several different scenarios where the Sox can suceed: Sign Oswalt, one of the dregs steps up, they get a starter mid-season (although it's difficult).
No need to despair.....yet.