The 40-year-old Mercker went on the 15-day disabled list for the second time this season with lower back pain. He had just been activated last week, getting a victory in relief Friday night.
The 23-year-old Herrera was one of two pitchers acquired from Texas in an offseason trade for outfielder Josh Hamilton. The other was starter Edinson Volquez, who is 7-2 with a 1.46 ERA.
In 26 relief appearances at Double-A Chattanooga and Triple-A Louisville this season, Herrera has a 1.85 ERA.