Montas was sent down to Triple-A Nashville on Monday, Susan Slusser of the San Francisco Chronicle reports.
Oakland will roll with a four-man rotation with a slew of off days coming up, so Montas was returned to the minor leagues following a start in Colorado on Sunday. The club will require a fifth starter versus Seattle on Aug. 14, but it remains to be seen whether Andrew Triggs (forearm) or Paul Blackburn (elbow) will be ready to come off the DL. In the meantime, Montas will continue starting at the Triple-A level.