Montas was optioned to Triple-A Nashville on Friday.
Montas will head back to the minors after getting tagged for seven runs across six innings in a spot start against the Mariners on Thursday. The A's only need a fifth starter once over the first half of September, allowing them to send Montas down for an additional position player for the time being. The right-hander should rejoin the A's once he's eligible, though it's unclear if he'll return to the starting rotation or be utilized out of the bullpen when he does.