Richards and the Angels are cautiously confident the injury isn't a sign of something bigger.
Richards missed most of last season because of a partially torn elbow ligament but chose rest and stem-cell injections over Tommy John surgery. He made it through spring training solidly but left his season-opening start at Oakland in the fifth inning Wednesday because of pain in his arm.
An MRI exam showed nerve irritation in Richards' biceps but apparently no problems with his elbow ligament.
"It's kind of the best thing that we could have hoped for, a mild biceps strain," Richards said. "I feel good. My elbow feels fine, shoulder feels fine. Must have been just some random thing."
Richards said he might start throwing again Tuesday, and he hoped to get "right back to pitching" after he comes off the DL.
"The smart thing to do would be to let it calm down, which is what we're doing," Richards said. "I feel totally fine, with the exception of that."
Richards is the Angels' most important starting pitcher when healthy, and his return is a key variable in their hopes of bettering last season's 74-88 record. He won 28 games over the 2014 and 2015 seasons but was limited to six starts last year.
"We're hoping just keeping him from throwing a baseball for about a week will get this behind him, and then we'll just see how he responds to his bullpens and when he's ready to go," manager Mike Scioscia said. "Considering where Garrett was last year, this is just a minor bump in the road."
Scioscia said Richards wanted to stay in and pitch when he felt the arm injury but was persuaded to be cautious.
"Once he picks up a baseball in a week, hopefully he's not feeling what he felt during the game, and it'll be behind him," Scioscia said.
The roster move is retroactive to Thursday, making Richards eligible to return April 16. The Angels don't need a fifth starter for another week, and Scioscia hasn't decided how they will fill the presumptive one-game gap in their rotation.
The Angels also recalled reliever Mike Morin before their home opener against Seattle.