The Athletics optioned Boyle to Triple-A Las Vegas on Wednesday.
Boyle has struggled since rejoining the big-league squad in mid-August, allowing 12 earned runs across 15.1 innings. He moved to the bullpen for his most recent outing, though it's unclear if he'll shift back into a starting role upon returning to Las Vegas. His roster spot will be given to Scott Alexander (shoulder), who returned from the injured list Wednesday.