Miles went on the disabled list on May 26 with a right shoulder strain. He was batting eighth for the Cubs in their game against Houston on Wednesday night.
Miles is hitting .204 with six doubles and four RBIs in 37 games.
Fox was recalled from Triple-A Iowa on May 27 and hit .417 with two doubles and two RBIs in eight games for the Cubs.
Fox led all minor leaguers with a .424 average when he was recalled. He has 17 home runs, 51 RBIs, 33 extra-base hits and an .881 slugging percentage in 41 games with Iowa.