Looks like Daisuke Matsuzaka is looking at a lengthy absence with a UCL sprain.
If you break a cold sweat looking at those last two words, you're not alone.
A UCL sprain is the injury that often leads to Tommy John surgery.
Let's go to SportsMD.com for the anatomy of the ligament:
The UCL is a band of tissue connecting the two bones (the humerus, or arm bone, and the ulna, the inside bone in the forearm) on the inside or medial part of the elbow.
When the UCL is injured -- either through trauma or repetitive stress -- it requires rest and rehabilitation. This can take around a month. If noninvasive treatment fails, the player is looking at reconstructive surgery and a 12- to 18-month absence.
For what it's worth, the Red Sox sound optimistic about Matsuzaka's case.