Gallen will be pushed back in the rotation by a few days due to a small cut on his thumb, Nick Piecoro of The Arizona Republic reports.
The Diamondbacks didn't initially provide a reason for scratching Gallen from his scheduled start Tuesday against the Astros, but he's apparently dealing with a minor thumb injury. Madison Bumgarner will now start Tuesday, while Merrill Kelly will take the mound Wednesday. In spite of Gallen's minor delay, he's expected to start at some point during the team's weekend series against the Mets.