Texas also placed left-hander Pedro Figueroa on the 15-day disabled list Wednesday because of left elbow inflammation. Figueroa is the 11th player on the Rangers' DL, although third baseman Adrian Beltre is set to return Friday from a left quadriceps injury.
Robertson is a career .304 hitter in the minors in 723 games in the Padres organization.
Figueroa left Tuesday night's game against Oakland after hitting Josh Reddick with his only pitch. He will undergo an MRI to determine the extent of his injury. Figueroa is 2-1 with a 4.00 ERA in 10 appearances this season.
Also, righty Neftali Feliz has left Triple-A Round Rock because of soreness and fatigue, but the Rangers said he is not injured.