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4/4/2013 2:44 A.M. ET

Freiman shines with two hits, RBI in MLB debut

OAKLAND -- Nate Freiman was still waiting for it to sink in.

"It hasn't hit me yet that this is the regular season, and these are Major League Baseball games," the rookie said Wednesday afternoon. "These past two nights, I've just felt like a fan with a really good seat."

Freiman was awarded an even better seat just hours later.

Resigned to spectator duties for the A's first two games of the season with as many right-handed opponents on the mound, the 26-year-old made his Major League debut at first base against Mariners lefty Joe Saunders for the third game.

"It's a big day," said Freiman, smiling. "I want to go out there and do what I did that got me here and not try to be someone I'm not. Hopefully we can get the win tonight, because I'd love to be on the field with everyone on a night like this, shaking hands."

The A's did, and Freiman collected two hits and an RBI in the effort.

"He was like a kid in a candy store in batting practice today, let alone in the game," manager Bob Melvin said. "Then you get your first hit and drive in a run and you make a nice play. This is one of those days he'll never forget. I told him before the game, 'The baseball gods will take care of you today.' He had a very nice day."

Claimed by the A's off waivers from the Astros less than two weeks ago, Freiman had not played above the Double-A level before Wednesday. It was there, in San Antonio with the Padres organization, where he posted a .298 average -- including a .336 clip with runners in scoring position -- with 24 home runs, 31 doubles and 105 RBIs in 137 games.

Freiman assumed he would be in Triple-A this year, "but then the Rule 5 Draft kind of changed everything," he said.

He went from the Padres to the Astros and, soon after, was welcomed into the A's organization, where he must remain on the big league roster or be exposed to waivers and offered back to San Diego. He offers Oakland the right-handed complement to Brandon Moss that went missing when Chris Carter was shipped to Houston.

"I'm beyond thrilled to be here right now," said Freiman, who left several tickets for a crowd that included his parents and brother. "I don't think I could possibly be more excited.

"I've definitely had better nights of sleep than last night."

Sizemore back on big league field for first time since '11

OAKLAND -- It had been nearly a year and a half since Scott Sizemore last played in a big league game, almost two years since he started one at second base.

But that's where he was Wednesday night, back in Major League action after missing all of 2012 while recovering from a torn ACL in his left knee. To say he was excited before the A's third game against the Mariners would be quite the understatement.

"Absolutely," he said. "I'm ready to get out there, get my feet wet again."

He went 1-for-4 with a double in his return.

"To get through Spring Training when you still have trepidation about an injury and get out on the field for a regular-season game, it's got to be rewarding for him, for a guy that put himself in this position again," manager Bob Melvin said.

Sizemore, 28, learned third base for the A's upon his move from Detroit in 2011, and that's where big things were expected from him in 2012, before he went down to injury on the first day of spring workouts last year.

Since, the A's have found a stable third-base option in Josh Donaldson, and they've asked Sizemore to return to his natural position at second base, where he won a job out of Spring Training -- for the first time since 2010 -- alongside platoon partner Eric Sogard.

"I was definitely relieved to make the team, obviously, and I'm just excited and happy and all of the things that come with being back on the field," he said. "It's been awhile since I broke with the team. I'm just hoping to make the most of my opportunity."

Mechanical issues dampen Cook's season debut

OAKLAND -- Ryan Cook's 2013 debut went rather awry, much like his performance in the Cactus League, where he finished with a 6.14 ERA. But the A's reliever is confident his next outing will look much different.

"I was just mechanically off," said Cook, who walked three and hit a batter in two-thirds of an inning in Tuesday's 7-1 A's loss to Seattle. "I knew what I was doing, and I couldn't make the adjustment. That's what made me more upset. I was just flying open with my front side, nothing staying closed, and I couldn't get anything glove side."

When Cook walked in a run, it snapped his 15-inning scoreless streak dating back to last season, which isn't even the longest of his young career. The right-hander started 2012 with a 23-inning scoreless streak en route to garnering an All-Star nod.

Cook was, at times, the A's best reliever last year, and he and lefty Sean Doolittle will be relied upon to set up for closer Grant Balfour this season.

"It felt really good to get back in there, even though the results didn't pan out the way I wanted," he said. "That just makes me want to get back out right away. I think I was just a little overamped, trying to do too much and make perfect pitches instead of just pitching how I pitch."

Worth noting

• The A's claimed right-hander Will Harris off waivers from the Rockies on Wednesday and optioned him to Triple-A Sacramento. To clear a spot on the 40-man roster, the A's designated righty Josh Stinson -- claimed off waivers from the Brewers last week -- for assignment.

Harris, 28, made his Major League debut with Colorado last August and compiled an 8.15 ERA in 20 relief appearances. He had two saves and did not allow a run in three Spring Training appearances before he was optioned to Triple-A on March 15. He was designated for assignment Friday.

• Rehabbing infielders Hiroyuki Nakajima (left hamstring) and Adam Rosales (intercostal muscle), both on the disabled list, were back on the field doing some work on Wednesday. Nakajima was scheduled to do some stretching and throwing, while Rosales was to take part in similar activities. Melvin said Rosales' next step is hitting off a tee.

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.