McCarthy has battled shoulder problems throughout much of his career. Last year he missed six weeks with a stress reaction in his right scapula, an issue he has faced in four of his past five seasons. Earlier this month he went 10 days in between starts because of soreness in his right shoulder.
"Things were getting progressively more sore and just really not recovering in between [starts]," McCarthy said. "It was at a point where I just felt it was best to shut it down and give it a little bit of rest and let it try to get itself back together. I think everything's fine in there. I think it just might be really fatigued."
This marks his seventh career trip to the disabled list and second in two seasons with the A's. He joins outfielders Coco Crisp and Yoenis Cespedes and third baseman Brandon Inge, among others, on the DL.
McCarthy said he had an MRI two weeks ago "when we were looking for something," but the test revealed no problem.
"It was completely clean," McCarthy said. "It leaves everyone pretty confident it's just really tired."
McCarthy plans to stop throwing for a few days and resume his throwing regimen once he's pain-free.
"He's got some shoulder stiffness, and based on his history we want to be proactive," A's manager Bob Melvin said. "We don't want to push it to a point that potentially he's out a significant amount of time like he was last year. So hopefully we can combat this with a two-week period."
McCarthy was 3-0 with a 2.43 ERA over his past four starts. He is 3-3 with a 2.95 ERA for the season.
Melvin said he'll have to call up a pitcher Tuesday from Sacramento to take McCarthy's spot in the rotation. Graham Godfrey, who was with the A's earlier this season, is the likely target.
Miller, who opened the season at Sacramento, was called up on April 25 and made six relief appearances for the A's, logging a 1.93 ERA. He was optioned to Sacramento on May 11. After being sent down, Miller made two appearances for the River Cats, allowing a run in 3 1/3 innings. Overall with Sacramento, he is 0-2 with two saves and a 2.92 ERA in 10 relief appearances.
Melvin said the A's called up Miller because their bullpen has been taxed lately and needed another fresh arm, especially a right-hander. Including Miller, the A's have three right-handers and five lefties in their bullpen.
"And his performance was good for us," Melvin said. "He's a guy that can give you length, he can match up. There are a lot of things he can do. He's really come a long way from Spring Training for a guy that really wasn't on the radar as far as the big league team, to pitch as well performance-wise. He pitched his way here and has done it again."
Miller said he's ready for any role the A's want him to fill.
"I've been doing a lot of two-inning stuff down there," Miller said. "I had the one outing up here that was three innings. It's whatever they need. If they need an inning, they need two, they need three. It's whatever they need."