Montas was optioned to Triple-A Nashville on Sunday.
The Athletics determined that Montas was in need of further refinement in the minors after he allowed 10 runs over 4.2 innings in his last four appearances. The hard-throwing right-hander had previously been developed as a starting pitching prospect, but since finding the strike zone has consistently proved to be an issue across multiple stops in the minors, it appears the Athletics are viewing him as a reliever over the long haul.