The Giants placed McDonald on the 15-day injured list Tuesday with a right elbow strain, Evan Webeck of the California Post reports.
McDonald made an early exit from his start Monday after suffering an injury while warming up for the fifth inning, and he'll now officially have to miss a few turns through the rotation. Tristan Beck will come up from Triple-A Sacramento to give the Giants some extra bullpen depth while the team sorts out its rotation plans.