"He feels pretty good," said Giants manager Bruce Bochy.
Affeldt's return comes at an opportune time for the Giants. Two of the Reds' top run-producers -- first baseman Joey Votto and outfielder Jay Bruce -- struggle against lefties. Votto is hitting .267 and Bruce .222.
Affeldt, the Giants' top setup man prior to the injury, is 2-3 with two saves in 18 appearances.