Junichi Tazawa, who was sent to see Dr. James Andrews on Monday, has a sprained ulnar collateral ligament, per Peter Abraham.
An injury to the UCL could require surgery, depending on the severity of the sprain. Takashi Saito, who tore his UCL in 2008, pitched effectively for the Red Sox last year despite never having surgery, but it is a torn UCL that is sending Joe Nathan to the DL with season-ending Tommy John surgery.
Saito was given an injection to help stabilize his elbow, but it would be surprising to see the Sox try a stop gap like that on a young pitcher if they expect he will need surgery at some point.
Luckily for Tazawa, a sprain is the less serious form of UCL injury, leaving the ligament damaged, but not completely torn. There is a chance he could recover through simple rest. It seems very unlikely that he will start his season any time soon, though.