I would not be surprised if the Sox put Pablo on the DL as sort of a "face-saving" measure. The situation is one they created, BTW.
But putting Pablo on the DL give the Sox an "out" if Shaw gets off to a really slow start this season. It won't look so bad that the Sox started Shaw if they had to, due to Pablo being on the DL.
I'm not saying that Shaw will stink. I actually think he will be fine there, though I'm not sure he can hold up for an entire season. For the record, my defense of Pablo has little to do with a lack of faith in Shaw.