Pauley gets the start tomorrow because that would have been his turn in the rotation in Portland. Lester pitched on sunday for the Pawsox so he is a no go and the fact that Pauley is already on the 40 man roster makes it a no brainer.
It's no really out of the blue considering it was his turn to start in the rotation for Portland and Wells is not likely to go to DL. He may only miss a start.