In his place, the Sox have activated Darnell McDonald, who, last I checked, is not a full-time pitcher. Due to yesterday's off day, it's conceivable they could take a turn through the rotation without a fifth guy, but that will catch up with them soon. They have off days on the 14th and 18th, however, so they could decide to just bite the bullet and try to make it a few weeks.
Of course, the other option is that tonight's starter in Pawtucket, Daisuke Matsuzaka, may return to the big club. That seems most likely, unless the team really feels he isn't ready to return at this stage from his injury. Keep an eye on updates from Pawtucket tonight to see if Daisuke starts and, if he does, how long he goes. If he only goes an inning or two, he'll be up by the weekend. If he goes a full start, maybe they have other plans in mind.