Castro was optioned to Triple-A Albuquerque on Saturday, Nick Groke of the Denver Post reports.
His most recent stint at the big league level was forgettable, as Castro gave up five runs on eight hits in just 2.2 innings of work (six appearances). The 21-year-old Castro can dial his fastball up to over 96 mph on average and still profiles as a closer long term, but it's certainly no lock he'll see save chances this season.