"He felt pretty good pitching, but there was enough soreness to get the MRI," Byrnes said.
The Padres will have Tim Stauffer take Erlin's turn in the rotation on Friday against the Cubs.
To replace Erlin on the roster, the team recalled reliever Blaine Boyer from Triple-A El Paso. Boyer had a 3.32 ERA in 18 games, all in relief. He last appeared in the big leagues on April 10 of 2011 with the Mets.
Erlin, 23, said that he has felt something in his elbow his last three or so starts. He's 3-4 with a 4.53 ERA in nine games this season, including eight starts. He picked up a victory in Colorado in his last start on May 17, allowing four runs in five innings.
"There wasn't really one specific time, I just felt it over the last few starts. Game situations it felt all right. But between starts I kind of felt it while playing catch," he said.
"[It's] a little bit of pain, in the throwing motion. I've just had some strains before. This is not quite as severe. Just taking the precautionary route -- hopefully take a little bit of time to save a lot of time."
The Padres have lost two starting pitchers to the disabled list in the last week -- Andrew Cashner, who also has soreness in his right elbow and now Erlin.
"It's the smart route to go. You think worst-case scenario you could be out for a year if something really happens. Just decided to take this route, and hopefully give it a little time and we won't have any more problems," Erlin said.
For a team that has been hit hard in recent years by pitchers needing Tommy John surgery, Erlin said he didn't let his mind wander down that dark path.
"I didn't freak out or anything, just thought it got to the point where I wanted to get it treated. There wasn't one pitch or outing where I said 'This is bad,' just kind of came up over time," he said.