From the AP:

Roger Clemens threw a bullpen session, pronounced himself ready to return to the mound Sunday night at Boston and revealed tests showed ligament damage in his sore right elbow. ...

Clemens said an MRI exam revealed ligament damage and internal bleeding in his forearm, adding that the muscle was "pretty nasty." The elbow problem might have stemmed from a blister on Clemens' foot that caused him to alter his mechanics.

He expects to pitch in the finale of the upcoming Red Sox series.