Castillo was outrighted Thursday to Triple-A Jacksonville, Jordan McPherson of the Miami Herald reports.
Castillo passed through waivers unclaimed after getting pushed off the Marlins' 40-man roster. Acquired from the Padres last month in exchange for cash considerations, the 27-year-old lefty reliever has made only three appearances in the majors since the end of his promising 2018 season in San Diego.