OAKLAND -- With a light rain drizzling the tarp covering the infield at McAfee Coliseum more than two hours before the scheduled start of an exhibition game against the visiting Giants, A's groundskeeper Clay Wood correctly assured Oakland manager Ken Macha that the game wouldn't be rained out.

"He must be involved with someone who's an even higher power than the higher power," Macha cracked.

Macha, it should be noted, often deflects questions regarding his team's roster to A's general manager Billy Beane by suggesting reporters check with "a higher power." And it was that power -- as well, perhaps, as Wood's meteorological prescience -- that shaped the 25-man roster the A's will take into Opening Night against the visiting Yankees on Monday.

Though the Friday trade of out-of-options outfielder Freddie Bynum appeared to lock the roster down, essentially assuring that on-the-trading-block outfielder Bobby Kielty would be on the team for the time being, the inclement weather forecast for the next few days forced a temporary change in thinking.

Kielty was optioned to Triple-A Sacramento after Saturday's game, opening a spot on the roster for recently acquired left-hander Brad Halsey.

In short, a rainout Monday or Tuesday would create a doubleheader Tuesday or Wednesday because the Yankees aren't coming back to Oakland this year. A doubleheader would push either Opening Night starter Barry Zito or Rich Harden, who is scheduled to pitch Tuesday, back a day. That would create the need for a spot starter the next time Zito's or Harden's turn in the rotation came, either Saturday or Sunday in Seattle.

Halsey, it appears, would get that start.

"They're expecting a doubleheader," Kielty said. "And they need a pitcher, not a hitter."

The way Macha explained it, the A's essentially had two options. They could be "proactive" and keep a 12th pitcher -- Halsey -- on the staff when they finalized the Opening Night roster, or they could have stuck with the plan to carry 11 pitchers and wait until the need for a sixth starter arose to make a corresponding move.

"You basically lose a player for eight games both ways," Macha said.

Kielty, the projected fifth outfielder for the A's, will have to stay in Sacramento for at least 10 days. It'll give him a chance to get regular at-bats, but he didn't seem in the mood to look at the bright side Saturday before leaving the clubhouse.

"I don't feel like I need the at-bats," he said. "I've got a week, two weeks without at-bats before and been fine. ... That's not the issue here."

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Though relievers Joe Kennedy and Kirk Saarloos both were members of the rotation at times last season, Macha said they wouldn't be asked to make any spot starts this early in the year because they haven't been "stretched out" as starters this spring and he doesn't want to "disrupt" the six-man bullpen.

Therefore, the candidates were Halsey, acquired last Sunday in a trade with the Diamondbacks, and lefty Juan Dominguez, acquired from Texas on Friday in the three-way trade that sent Bynum to the Cubs.

"Halsey has been up to 80 pitches [this spring]," Macha noted. "But I would think Dominguez has, too."

Unlike Halsey, Dominguez was not with the A's on Saturday. He'll start the season as part of the starting rotation in Sacramento.