The 12-year veteran will throw another session Friday, and he said the next step would be to work on fielding drills.
"There's probably not going to be a ton of appearances," Affeldt said of his potential return. "But just to be able to come back and end the season healthy is going to be a big thing for me going into the offseason."
In 38 relief appearances this year, Affeldt has posted a 3.51 ERA.
In other injury news, righty reliever Sandy Rosario is battling lower-back soreness. Manager Bruce Bochy said Rosario told him he's feeling much better than he did Monday. But Bochy -- given an influx of arms with the addition of September callups -- said he'll likely shy away from using Rosario.
AJ Cassavell is a reporter for MLB.com. Follow him on Twitter @ajcassavell. This story was not subject to the approval of Major League Baseball or its clubs.