Mengden was optioned to Triple-A Nashville on Sunday.
Mengden made two turns in the rotation during his stay with the big club, covering eight innings and surrendering nine runs on 13 hits and three walks. The right-hander's pair of poor outings may have left the Athletics reluctant to give him additional starts in the majors anyway until he showed improvement at Triple-A, but Mengden's demotion also signals that the team might be anticipating Jesse Hahn's (triceps) return from the disabled list later this week.