Joe Kelly felt some tightness in his right biceps as he was warming up Monday, which led him to leave his start in the third inning.
"As the game progressed it got worse than I thought it was," said Kelly, who was limited to just fastballs against the New York Mets because of the tightness.
Kelly said he was not concerned because he has had biceps tendinitis before.
"Something we looked over in the training room," he said. "Everything checked out. A little biceps restriction. It's something we're going to work on and try to let it settle down a little bit, then continue on throwing."
The Boston Red Sox haven't indicated how long they'll rest Kelly.
Via Gordon Edes/ESPN