"That's probably fair," manager Bob Melvin added. "When [he'll pitch,] we're not sure."
The 24-year-old Anderson, recovering from a right oblique strain he suffered two weeks ago, threw a bullpen session Monday and came out of it feeling a bit tender on Tuesday, which wasn't unexpected.
Should the A's play in the American League Wild Card game on Friday, they will have to decide whether it will be Anderson or rookie Tommy Milone on the mound.
"We're not going to rush him into anything, so he's going to have to be very confident in what he's doing," Melvin said.
"You really can't simulate in a bullpen what you do out there in a game. If anybody throws as significant bullpens and goes at it pretty hard, it's Brett. You can't impede the progress he's made right now by trying to rush him too quickly."