The Diamondbacks optioned Castellanos to Triple-A Reno on Tuesday, Jesse Friedman of GoPHNX.com reports.
Castellanos threw 40 pitches over three innings in a relief appearance in Monday's loss to the Royals, so he wasn't going to be available to pitch for at least a day or two. He's clearing out to make room on the roster for Jordan Montgomery, who returns from the 15-day injured list to start Tuesday in Kansas City.