Bush and Gallo collided while chasing a popup in the eighth inning of Texas' 3-2 home loss to the Chicago White Sox. Bush bruised his right knee and Gallo sustained a non-displaced nasal fracture and a swollen upper lip.
Outfielder Ryan Rua was recalled from Triple-A Round Rock to fill the roster spot of the 31-year-old Bush, who has a 3-4 record with 10 saves and a 3.04 ERA in 49 appearances.
Rua has batted .202 with three homers and 11 RBIs in 45 games with the Rangers this season.
Texas also optioned infielder Phil Gosselin to Round Rock and selected the contract of right-hander Nick Gardewine from Double-A Frisco to take Gosselin's place on the 25-man roster.
Gosselin entered as a defensive replacement Sunday after Gallo's exit in the eighth inning. He is batting .146 with two RBIs in 29 games with Pittsburgh and Texas this season.
Gardewine has posted a 1-2 record with four saves and a 2.41 ERA in 30 relief appearances at Frisco.