Castillo was optioned to Triple-A Louisville on Wednesday, one of 16 players cut from the Reds' major league camp, MLB.com reports.
While Castillo was impressive in spring training, the plan all along was for him to begin the season in the minors. "Probably the first thing is that he hasn't done anything to take our optimism away," Reds manager Bryan Price said. "What we had to do, considering innings and opportunities we had for length, is we had to pare it down to who are the most likely. We had to rank them in order to give them regular time and regular innings. I just felt with his limited Double-A time that it made more sense for him to start the season in player development and maybe be one of those quick-jump guys should we need him early in the year."