Cobb pitched seven innings in a 1-0 win over the Cincinnati Reds on Saturday. He's gone 15 1/3 innings without allowing a run.
The Rays called up left-handed reliever Jeff Beliveau from Triple-A Durham to take his spot on the roster.
Beliveau pitched 22 games in relief for the Cubs in 2012 and one for the Rays last season. He threw one perfect inning during a 12-4 loss to the Reds and was sent back to the minors. The Rays plan to call up another reliever on Monday.
Left-hander Erik Bedard was added to the roster on Sunday and will take Cobb's spot in the rotation for now.
Left-hander Matt Moore went on the 15-day DL earlier in the week with a sore elbow. The Rays moved left-hander Cesar Ramos from the bullpen into the rotation to take his place -- Ramos started Sunday against Cincinnati and took the loss, pitching only into the third inning.
Ramos and Bedard competed for a spot in the rotation during spring training, so they were ready to be starters when the season began.
Cobb felt some tightness in his side during the game on Saturday, but he was throwing so well that the Rays thought nothing of it.
"I'm not exactly sure when it happened," manager Joe Maddon said. "We thought it could have been dehydration."
He was examined after the game and sent back to Florida for an MRI that confirmed that he has a strained muscle. The Rays put him on the disabled list right away because they knew he'd miss one or two starts anyway.
"If we do everything right, there's a chance we could have him back for that third start," Maddon said.