McCullers was diagnosed with a mild elbow sprain, Brian T. Smith of the Houston Chronicle reports.
McCullers isn't expected to throw for at least two weeks, so he'll likely be out until at least the latter end of August. The team will want to be careful with McCullers, as he'd been drastically improving in recent starts and owned a 2.57 ERA and 12.9 K/9 over his last five outings.