Consider this...
With Dmats throwing today and Beckett throwing tomorrow, it is reasonable to assume that DiceK is the beginning of a new turn of the rotation. If the sox lose tonight, it is very possible that the sox will be under .500 in this turn of the rotation. This is why you need a new pitcher. Because you are relying on Beckett and DMats to win and then hoping to slug your way to a victory in one of the following 3 games that the shitbums pitch. If either loses, then you are doomed to a poor turn.