Montas was optioned to Triple-A Nashville on Tuesday, Susan Slusser of the San Francisco Chronicle reports.
The A's won't need Montas to make another start until after the All-Star break, so they will take this opportunity to add Ryan Dull to the bullpen for the final six days of the first half. It is unclear if he will start for Nashville or if the A's will use this as an opportunity to shut him down for a week or 10 days. He could rejoin the rotation against the Giants or against the Rangers in the second half.