SEATTLE -- Rallying behind starter Bartolo Colon, the A's locked down home-field advantage for the first round of the postseason Friday night with an 8-2 victory over the Seattle Mariners, but in the process Oakland may have lost Yoenis Cespedes for the short term.

Cespedes, in the lineup in left field for the first time since Sept. 13 because of right shoulder soreness, got two at-bats and came out of the game with the shoulder a problem once again.

This time, however, the pain popped up while Cespedes was swinging. For the past few weeks he's been able to swing but not throw. It's unlikely Cespedes will play in either of the last two games of the regular season.

"I don't know what to tell you," manager Bob Melvin said of the Cespedes turn of events. "There's a good chance he's not going to be out there (Saturday)."

Colon, who's likely to be the A's starter come Game 1 of the playoffs, allowed Seattle two solo home runs and not much else over six innings. He ended the regular season with an 18-6 record and career-best 2.65 ERA. If Detroit's Anibal Sanchez (2.64) gets roughed up even a little Saturday in Miami, Colon would win the American League ERA title.

"I never dreamed of anything like that," Colon said through interpreter Ariel Prieto. "More surprising to me than the (18 wins) is the ERA."

Colon's victory, coupled with the Tigers' 3-2 loss in Miami, means the A's will host Games 1 and 2 of the A.L. Division Series starting next Friday. Game times haven't been announced, and the opponent is yet to be determined.

"That's what we've been playing for (since locking down the A.L. West)," Brandon Moss said. "Home field is very, very important to us. It's a tough place for other teams to play."

Moss gave Colon everything he would need with a first-inning, three-run homer off Mariners ace Felix Hernandez, who had limited Oakland to no runs in 142/3 innings in two previous starts this season.

"I was looking in the zone for a fastball, and I got one," Moss said. "He's such a good pitcher. That was maybe the one bad pitch he made all night."

Oakland is likely to face the Tigers in the first round of the playoffs, but if the A's win their last two games and the Boston Red Sox lose their last two, the A's would face the wild-card team instead. That competition is down to Tampa Bay, Cleveland and Texas.

Colon was also backed by a two-run, pinch-hit homer from Derek Norris in the seventh. It was the third pinch-hit homer of the season for Norris, matching the club record. Four other A's hitters have accomplished that feat, with Jeff Burroughs the last in 1982.

"Those may be the only three hits I have (as a pinch hitter)," Norris said. "It's a tough job. But you're up there to drive in runs."

  • Nate Freiman took batting practice before Friday's game, and it seems the first baseman's abdominal problems are behind him and he'll be able to be on the postseason roster.

  • It's beginning to look as if Kurt Suzuki won't be on the postseason roster. The veteran catcher has started just one game since Sept. 13.

  • Coco Crisp's four hits Friday give him a .311 average in his past 33 games and a .263 average overall.

  • The A's players met before the game and voted on how postseason shares will be divided.

  • John Jaso will continue to work with the instructional league team through the weekend, then will join the A's early next week in Oakland. There's still a remote chance Jaso, who has missed all of August and September with concussion problems, could make the postseason roster, but only as a D.H./pinch hitter and not as a catcher.

  • The A's have not made all their roster decisions yet, and Melvin said what happens this weekend might have some impact on the first-round roster. He said he might be ready to announce the starting pitcher for the A.L. Division Series on Tuesday, although Colon is expected to get the call. The manager and his staff will meet with the front office Monday to discuss options while the players get the day off.

    For more on the A's, see John Hickey's Insider at blogs.ibabuzz.com/athletics.

    SATURDAY'S GAME

    A's (Jarrod Parker 12-7)
    at Seattle (Brandon Maurer 4-8),
    1:10 p.m. CSNCA