The Diamondbacks designated Castellano for assignment Sunday, Alex Weiner of ArizonaSports.com reports.
The 26-year-old was optioned to Triple-A Reno on Thursday and has now lost his place on the 40-man roster. Castellanos posted a 5.23 ERA in seven outings for the Diamondbacks, and he could remain with Reno if he passes through waivers unclaimed.