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4/30/2014 8:10 P.M. ET

Norris keeps hitting, but role unlikely to change

ARLINGTON -- Derek Norris received his second straight start behind the plate against the Rangers on Wednesday and is expected to be there again Saturday in Boston.

That's because the A's are facing left-handed starters on those days.

Manager Bob Melvin isn't ready to stray from a platoon at catcher, even though Norris is swinging a hot bat. The catcher entered the day batting .481 (13-for-27) over his last nine games, and his .365 season average is the best in the American League among batters with 50 or more plate appearances.

" He's been terrific, he really has, from the beginning of the season until now," said Melvin. "We do it a certain way here, but he's doing very well. The better you do, the more reps you end up getting."

For now, John Jaso will continue starting against most right-handers, while Norris remains prepared in a part-time role. Jaso is hitting .232 but .250 against right-handers this year.

"The most important thing is try to find the barrel and not try to do too much it," said Norris. "I'm always striving to get better as a ballplayer, and whatever that is, whether it's facing lefties or coming in to pinch-hit, staying mentally prepared and staying mentally strong is the biggest thing for me."

Griffin has Tommy John surgery

ARLINGTON -- As expected, A.J. Griffin's elbow procedure in Houston on Wednesday resulted in Tommy John surgery.

The A's right-hander, initially shut down during spring with flexor tendinitis, is expected to miss 12-18 months.

Griffin is the second A's starter -- Jarrod Parker underwent his second Tommy John surgery in late March -- and one of over a dozen pitchers around the Majors to undergo the operation in the last month.

The club's head trainer, Nick Paparesta, said Griffin's procedure went extremely well, noting that no cartilage needed repair and the ulnar nerve did not need to be moved. Dr. Thomas Mehlhoff, who also performed A's reliever Fernando Rodriguez's Tommy John surgery with successful results, was able to use the Palmaris tendon from Griffin's right wrist as a graft.

"The joint looked great," said Paparesta. "Dr. Mehlhoff was very excited, encouraged about the graft, and he feels pretty good about how it came out."

All of these things will help Griffin's recovery, as will the fact he's still just 26-years old. Moreover, the mental challenge that comes with such a lengthy rehab will be aided by Parker's company.

Griffin won 21 games for the A's the last two years, posting a combined 3.60 ERA.

"I think it's going to be great for A.J., because Jarrod's been through it," said Paparesta. "So when A.J. has all those questions, Jarrod will be there to help with a lot of them."

Paparesta's staff hopes to use the time to help Griffin improve his overall physical condition.

"We do want to take a holistic approach," he said. "We want to find A.J.'s weaknesses, if his core strength is where it needs to be, if his rotator cuff strength is where it needs to be. Does to he need to work on his lower half? Does he need to work on his upper half, and does he need more flexibility? And then we put all of that into the rehab program. It's not just come in and do two hours of therapy on the elbow and go home. It's about putting together the best package we can for him."

Cook playing catch-up in bullpen

ARLINGTON -- Ryan Cook hasn't been forgotten as a setup man. The A's right-hander has simply been playing catch-up.

Cook pitched the sixth and seventh innings of a game the A's gained a significant lead in the early going in Texas on Tuesday, but that's not because his role has changed.

"If I didn't have Luke [Gregerson] available on a particular day, it's easy to stick him in the eighth inning," said manager Bob Melvin. "One of the things we wanted to do is stretch him out, because he hadn't had that. He really didn't go through what everyone else did in Spring Training.

"To be able to get him those two innings yesterday was important."

Cook spent all of Spring Training nursing a sore shoulder back to health, and he didn't make his season debut until April 7. Since, he's allowed just two hits in 9 1/3 innings but has also walked seven, though he's currently riding a six-game scoreless streak.

Worth noting

• Infielder Hiro Nakajima, signed by the A's to a two-year, $6.5 million deal at the end of 2012, was transferred from Triple-A Sacramento to Double-A Midland on Wednesday.

The Japanese import hasn't panned out as the A's hoped. Initially pegged as their potential starting shortstop for 2013, Nakajima struggled last spring and began the season on the disabled list. He spent the entire year in Sacramento and, through 12 games with the River Cats this year, was batting just .128.

Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.