OAKLAND — The A's have lost reliever Ryan Cook to at least 15 days on the disabled list thanks to a muscle strain in his right forearm.

Cook had an MRI Thursday and the news is good -- it's a simple forearm strain and nothing involving surgery.

"When I left here (Wednesday), I was 100 percent confident this wasn't a Tommy John (surgery) thing,'' Cook said. "And that's what the MRI showed. So I shouldn't be out long.''

At the same time the club has added a starting pitcher. Drew Pomeranz is officially in the Oakland rotation after making a five-inning scoreless start in the second game of Wednesday's doubleheader.

The opening for Pomeranz was created with the club optioning right-handed starter Dan Straily to Triple-A Sacramento.

The two open spots on the roster will be filled by right-handed reliever Fernando Rodriguez and left-handed reliever Joe Savery, both of whom have been pitching for Sacramento.

The fact that Pomeranz threw five two-hit, zero-walk innings Wednesday gave the A's the option to send down Straily for some work on his command. He'd walked 15 in 381/3 innings, and he'd been pitching behind in the count in much of the last five games.

"He needs to work on refining his fastball command and getting ahead,'' manager Bob Melvin said. "We have seen him have success here before. We feel like best avenue for him is to go down and work where there's less pressure tan you get at the big league level. He's still a guy we expect big things out of at the big league level.''

Wednesday's start was Pomeranz's first start of the season after working as the long man out of the bullpen.

"In the past he's had a little trouble throwing strikes, but that hasn't been the case here,'' Melvin said. "He has a plus fastball and a plus breaking ball. He used his change-up in the start the other day. He's pitched his way into this role.

Cook missed all of spring training and the first week-plus of the season while recovering from shoulder tendinitis. He came out of the 10th inning of the first game Wednesday holding his forearm. The A's, locked in a 4-all tie at the time, saw the Mariners score two runs later in the inning as Cook took the loss. He goes to the DL with a 2.92 ERA, but had a 0.96 ERA in his first nine games.

"As soon as I'm asymptomatic I'll be back into baseball activities,'' Cook said. "I don't think it will be too long.''

Rodriguez, who missed all of 2013 because of Tommy John-style ligament replacement surgery, began an injury rehabilitation assignment on April 3, appeared in eight games, then was taken off the DL and kept in Sacramento. He was 1-0 with a 0.73 ERA in his 11 games with the RiverCats.

"The velocity is still there,'' Rodriguez said of his arm post-surgery. "I'm a different pitcher though because I spent last year on the bench watching games. I've seen how to be effective without throwing 20-15 pitches an inning.''

Savery, claimed off waivers this spring from Philadelphia, went 4-0 with a 3.86 ERA and one save in 10 appearances with Sacramento.

"He gives us a third left-handed option in the bullpen with Pomeranz going into the rotation,'' Melvin said.