The Padres optioned Castillo to Triple-A El Paso on Sunday.
Castillo will move up a rung on the minor-league ladder after spending time with High-A Lake Elsinore and Double-A San Antonio a season ago, producing a 2.88 ERA, 1.28 WHIP and 59:26 K:BB in 56.1 innings across the two levels. He'll have a good chance of making his Padres debut at some point later in the season after San Diego added the lefty to its 40-man roster in November.