The move was announced after Milwaukee's 5-3 loss to Cincinnati on Thursday. The loss eliminated the Brewers from the National League playoff race.
Rookie Jimmy Nelson (2-8) will start the opener of the three-game series with the Cubs to wrap up a once-promising season. The Brewers were 16 games over .500 and in first place on Aug. 19, before a 10-23 collapse dropped them out of the NL Central race.
Garza finished 8-8 in his first season in Milwaukee. The veteran had been sidelined for most of August with a strained rib cage muscle.