"It just flared up to the point where I just couldn't do it, couldn't deal with it," Walden said. "My biceps is just very, very weak.
"My shoulder was weak, and then my biceps started going."
For Tuesday, the Angels will have to make another roster move to call up their starting pitcher against the Tigers -- likely Garrett Richards -- and it'll probably be Calhoun or infielder Andrew Romine going down.
The Angels don't believe Walden's injury is serious. He'll go through normal rehab for now, is expected to travel with the team to Detroit after Sunday's series finale against the Yankees, and Walden -- eligible to be activated as early as July 24 -- expects to start throwing again "soon."
"It doesn't appear to be [serious], but we want to make sure we get on top of it," Angels manager Mike Scioscia said.
"I think every pitcher's going to pitch with a little bit of inflammation. There's not a pitcher in that room that feels 100 percent, and Jordan's pitched with some things from time to time. But when it gets to the point where you don't feel you can comfortably execute your pitches, you need to take a little step back."
With Walden gone, it'll be up to LaTroy Hawkins, Jason Isringhausen and Kevin Jepsen to step up as a bridge to Scott Downs and Ernesto Frieri.
Walden, 24, was an All-Star who posted a 2.98 ERA and racked up 32 saves in his rookie year last season. This year, though, he was removed from the closer's role in late April and continued to be erratic as one of Scioscia's middle relievers, compiling a 3.86 ERA, a 1.57 WHIP and a 1.94 strikeout-to-walk ratio.
"I should be fine," Walden said. "It's just a little bump in the road."