"It was evident yesterday that he's not quite right," Giants manager Bruce Bochy said. "It's time to give him a break."
Affeldt has been a key piece to the Giants' bullpen since signing with the team as a free agent in November 2008, but he has struggled to find consistency this season.
"He's had a little trouble getting up to his normal delivery at times," Bochy said.
The southpaw's 5.96 ERA in 22 2/3 innings this year is his highest total for a single season since posting a 6.20 ERA in 97 1/3 innings split between the Colorado Rockies and Kansas City Royals in 2006.
"It's kind of a gradual thing," Bochy said. "It's not like he couldn't pitch or was hurting a lot. I think he's more tired than anything."
Affeldt has struck out 5.2 batters per nine innings in 2015, his lowest since striking out 4.4 batters per nine in 2006, but his absence leaves Javier Lopez as the only left-handed reliever in the Giants' bullpen.
The team also has Jake Peavy and Matt Cain nearing returns to the starting rotation, leaving Bochy in a tough spot with his roster decisions.
"We have some moves to make here with the starters," Bochy said. "We'll talk about this as we make a couple more moves."
Broadway, who threw 4 2/3 innings in his first stint with the Giants earlier this month, has gone 1-0 with a 1.30 ERA in 27 2/3 innings with Sacramento this season. He's allowed one run in four Major League appearances, all of which have come in June.
"Broadway was throwing well, so that's a simple [decision]," Bochy said.
Oliver Macklin is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.