Howard was placed on the 15-day injured list Wednesday with right shoulder impingement syndrome, Richard Justice of Texas Monthly reports.
Howard surrendered six runs (five earned) over 3.1 innings during his last start Sunday, and he'll now miss at least a couple turns through the rotation due to the injury. The 26-year-old will be eligible to be reinstated Aug. 23, though it's unclear if he's expected to be fully recovered by that point. John King was recalled from Triple-A Round Rock in a corresponding move.