"Well, it's another standard 'me injury' on an off day," Affeldt told reporters Tuesday. "We were kind of playing around, and I just lost my footing. ... It's unfortunate. I'm frustrated. I feel bad for what happened -- it's a little embarrassing -- but I wasn't doing anything stupid."
He said he heard a pop in his knee while stepping off a platform in a lake at his home in Spokane, Washington.
Said manager Bruce Bochy: "Jeremy being Jeremy. I shouldn't say it, because he'll get on me, but he's not the most athletic guy."
Affeldt is 2-2 with a 5.46 ERA in 44 games this season. The Giants announced the move before their game against the Chicago Cubs.
The Giants recalled right-hander Mike Broadway from Triple-A Sacramento to take Affeldt's place. Broadway appeared in six games with the team earlier in the year, recording a 3.68 ERA.