The Royals optioned Mills to Triple-A Omaha on Wednesday.
He'll cede his spot on the 26-man active roster and in the bullpen to right-hander Joel Payamps (personal), who was reinstated from the bereavement list. Mills turned in a 5.66 ERA and 1.66 WHIP while collecting four holds over his 18 appearances following his July 1 call-up from Triple-A.