The Diamondbacks designated Castillo for assignment Monday.
Castillo will lose his spot in the Arizona bullpen with the Diamondbacks activating right-hander Kendall Graveman (back) from the 15-day injured list in a corresponding move. After getting called up from Triple-A Reno on May 1, Castillo filled a low-leverage relief role and gave up eight earned runs on 10 hits and three walks in 6.1 innings across five appearances. The 29-year-old lefty is likely to clear waivers and will have the option of either accepting an outright assignment to Reno or electing free agency.