OAKLAND -- A's right-hander Ryan Cook suffered a forearm strain in the first game of a doubleheader against the Mariners on Wednesday and will undergo an MRI exam during the team's off-day Thursday.

Cook said early Wednesday evening he is "really optimistic" the injury isn't serious, emphasizing his symptoms are not in line with those experienced by Tommy John patients.

"No pain in those areas," he said. "Seems to be strictly in my forearm, just tightness.

"It's not as bad as you guys think it is, I can tell you that. At least we think right now."

Charged with the loss after allowing two runs in the A's 6-4 loss to Seattle, Cook appeared to be in serious pain after striking out Corey Hart on a slider in the seventh inning. He exited the game immediately, with the A's calling on Dan Otero to finish the inning.

"I felt it on the last two pitches to Hart," he said. "I threw a slider, and my forearm felt like it tightened up on me. I stretched it out to where I could throw again and threw him a slider that struck him out, and that's when it really locked up, and it was about as much as I could take.

"I've never had anything happen like this before, so I have nothing to compare it to. Right now it's tight. I literally have no other way to describe it."

Cook missed all of Spring Training because of shoulder tendinitis but was back pitching for the A's on April 7. He's allowed four earned runs in 12 1/3 innings.

Should he need to go on the disabled list, the A's could potentially turn to righty Fernando Rodriguez, who has allowed just one earned run in 12 1/3 innings for a 0.73 ERA for Triple-A Sacramento.