Castillo was optioned to Triple-A Reno on Sunday.
Castillo was acquired by the Diamondbacks in December and had a solid showing in spring training with a .270/.325/.459 slash line in 17 contests, but it wasn't enough to give him a spot on the Opening Day roster. The 25-year-old had a .633 OPS over 96 games in his first taste of MLB action with the Pirates last year.