Alberto was sent down Thursday, a day off for the Rangers. Mendez will start Friday night in the opener of a three-game series at home against the Colorado Rockies.
Mendez is 0-6 with a 5.26 ERA in 10 starts for Round Rock. His 10 previous MLB appearances have all been in relief. He has appeared in only one game for the Rangers this year.
Struggling starter Matt Moore (1-5, 7.88) will go to the bullpen for now to work on his delivery.