Beckett, who had just come off the list, aggravated a blister on his throwing hand Monday in a 3-1 loss to Pittsburgh.
A team doctor said he has a "skin tear" on one of his fingers.
"I've never had a blister that hurt this bad, then to lose this game, it (stinks)," Beckett said. "It's tough, but what the hell am I going to say about it?"
Beckett, the 2003 World Series MVP, is 4-5 with a 3.89 ERA in 13 starts this season.