The Diamondbacks optioned Gilbert to Triple-A Reno on Wednesday.
Gilbert's demotion comes as little surprise after he was blasted for seven runs (six earned) on five hits and two walks over 1.2 innings in Tuesday's 14-8 loss to the Reds. The Diamondbacks still have a need for a No. 5 starter while Humberto Castellanos (elbow) is on the shelf, but the newly signed Dallas Keuchel or Caleb Smith look like the frontrunners for the job when Gilbert's next turn in the rotation comes up this weekend in Philadelphia.