The fifth spot is up for grabs, with three contenders still in the running: Woody Williams, Jack Cassel and Chris Sampson.
"That could change," Cooper warned. "As the next week unfolds, that's how it's looking right now."
Chacon's sterling five-inning performance against the Braves on Wednesday night solidified his spot in the rotation. Williams has struggled for much of the spring, and his status with the team is both unstable and unclear. He'll likely receive two more starts through the remainder of the exhibition season -- on Sunday against the Braves in Lake Buena Vista, Fla., and next Friday against the Tigers at Minute Maid Park in Houston.
The two candidates who do not make the rotation will receive consideration as a long reliever in the bullpen. Asked if Williams could possibly make the team in that capacity, Cooper was non-committal.
"It's a possibility," Cooper said. "You don't want to say, 'Yay' or rule it out. We'll see how it works out and make a decision when that time comes."
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.