San Francisco recalled right-hander Joe Martinez from Triple-A Fresno to take Affeldt's place.
Affeldt was injured while warming up in the bullpen Friday night against the Arizona Diamondbacks. An MRI showed a slight tear of the oblique, but Giants manager Bruce Bochy hoped he'd be able to return after the 15 days was up.
Affeldt is 2-3 with a 4.11 ERA and three saves this season, and has not allowed a run in 12 of his last 13 outings. Without him, the Giants don't have a left-hander in the bullpen.
Martinez was 0-1 with a 4.00 ERA in three games in one previous stint with the big-league club this season.