The Guardians optioned Brennan to Triple-A Columbus on Tuesday, Mandy Bell of MLB.com reports.
Brennan was sent to the minors as the corresponding transaction for the addition of trade-deadline pickup Lane Thomas to the roster. He easily leads the Guardians with 50 starts in right field this season, but Brennan's playing time is now slated to go to Thomas. The 26-year-old Brennan is slashing just .233/.291/.379 over 247 plate appearances this season.