Beltre suffered a sprained left thumb while sliding into second base Sunday against the Boston Red Sox. He is expected to sit out at least two weeks.
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.
Gallo, 21, is hitting .314 with nine home runs and 31 runs batted in for Frisco. His home runs have come in the last 23 games.
Gallo was the 39th overall selection in the June 2012 amateur draft.
Mariners send Chris Taylor to the minors
The Seattle Mariners sent shortstop Chris Taylor to the minors.
Taylor made 18 starts and appeared in 20 games since being called up May 6 but was hitting .159 with one RBI and 19 strikeouts. He was hitless in his last 12 at-bats.
Seattle Manager Lloyd McClendon announced that left-hander Mike Montgomery will start Tuesday against the New York Yankees, with James Paxton on the 15-day disabled list. A move to clear a roster spot for Montgomery will be made before the game.
Miami Marlins right-hander Jose Fernandez took another step in his recovery from elbow ligament-replacement surgery, throwing three hitless innings against minor league players in an extended spring-training game in Jupiter, Fla.
Fernandez faced 10 batters and struck out seven, including the last six. He walked one and threw 42 pitches, with the radar gun topping out at 97 mph.
Right-hander Jarred Cosart, recovering from a bout of vertigo, started against Fernandez and threw 60 pitches.
Fernandez is expected to begin a series of rehab starts with Class-A Jupiter on Saturday. Barring a setback, he could join the Marlins rotation around July 1.
Cosart probably will make at least one rehab start in the minors before rejoining the Marlins.