The Red Sox said Wednesday that Matsuzaka has elbow tightness "consistent with a sprained ulnar collateral ligament." A sprained UCL often, but not always, results in Tommy John surgery, and it's possible Matsuzaka was shut down while the injury is manageable without surgery.
The club said for now, Matsuzaka will rest for two weeks and be reexamined.
Even if no surgery is necessary, Matsuzaka will need time to rebuild arm strength, and manager Terry Francona told reporters it's unlikely Matsuzaka would be ready to begin throwing after the upcoming two-week rest window.
Matsuzaka gave up five earned runs and allowed 12 baserunners in just 4 1/3 innings of his last start, on Monday against the Baltimore Orioles. While he has a 3-3 record and a 5.30 ERA, Matsuzaka has had a couple of decent outings this month, and his loss would still be significant for the Boston rotation.
Matsuzaka is due to make $10 million this year and in 2012, after which he is eligible for free agency. Should Tommy John surgery be required, he would miss all of this season and a good portion of the 2012 season.
The Red Sox will man the back end of the rotation and the long relief roles with some combination of veteran knuckleballer Tim Wakefield, right-hander Alfredo Aceves and Michael Bowden. Fellow starter John Lackey also is on the DL with an elbow injury.