OAKLAND -- Eric Patterson has big plans for the last two weeks of the season, starting with a determination to transform himself from a fill-in to a regular for the Athletics.

Patterson has been the primary replacement for injured second baseman Mark Ellis, starting 17 of the past 18 games. Cliff Pennington will also see time at the position, including getting the start at second Friday night against the Rangers.

Since hurting his right shoulder Aug. 21 when he landed hard after Seattle's Jose Lopez broke up a double play, Ellis made just two appearances. On Thursday, Ellis said he would miss the rest of the season because of the injury. Ellis hit .233 with 12 home runs and 41 RBIs in 117 games.

Patterson, acquired in July by the A's as part of a six-player trade that sent pitcher Rich Harden to the Cubs, has shuttled between Triple-A and the Major Leagues over the past two years. He hopes to use the last 15 games of the season to strengthen his foothold in Oakland.

"It's definitely an opportunity to prove that you can go out and play second base at the big league level," Patterson said. "I know that for me, that's kind of been a question mark, whether I can stay there or not. But I don't buy into that. I feel I'm capable. I'm not going to put any added pressure on myself. I'm going to take it for what it is and go from there."

Patterson, one of 15 rookies on the A's roster, has also played left field. He welcomed the chance to return to second base, the position he has played for most of his career.

"I definitely feel comfortable there," said Patterson, 25.

Finding a better comfort zone at the plate is also a priority for Patterson, and that figures to come with more playing time. He's batting .141 (11-for-78) the first time he sees a pitcher in a game and .275 (14-for-51) after that.

"I've hit my entire career, but right now I'm not swinging the bat like I'm capable," said Patterson.

Manager Bob Geren said Patterson is starting to show signs that he's able to connect more consistently.

"He hasn't really linked any big offensive games together, but he's shown some flashes, driven some balls in the gaps," Geren said. "Sometimes, it takes a while for a guy to get in a groove. He's had 90 at-bats. Give guys 100-150, and it's a pretty fair test of what they can do."

Geren added that Patterson also bolsters the A's overall team speed, along with Pennington and leadoff man Rajai Davis.

"It's nice to have different avenues to score some runs," Geren said. "Home runs are way down as a team, and that speed has helped us win some games."

Meanwhile, Geren said doctors are still trying to determine whether Ellis should undergo surgery to repair the damage in his shoulder. He also said he would continue to lobby for a Gold Glove for Ellis.

"He's the best second baseman in the league, and he deserves to win it," Geren said. "I hope being hurt in the last two weeks doesn't hurt his chances."