Beltre was placed on the 15-day DL on Monday with a sprained left thumb, and the Rangers say they plan to purchase Gallo's contract from Double-A Frisco on Tuesday before their series opener at home against the Chicago White Sox.
The 21-year-old Gallo is hitting .314 with nine homers and 31 RBIs in 34 games for Frisco. All nine of his homers have come in the past 23 games.
Gallo was a supplemental pick, 39th overall, in the June 2012 amateur draft.
Beltre is expected to miss at least two weeks after spraining and cutting his left thumb while sliding into second base Sunday.