Castillo was added to the Rays' 40-man roster Monday and is protected from the Rule 5 draft.

The 23-year-old righty leveraged a strong start at Double-A Montgomery into a promotion to Triple-A Durham in June. He was used almost entirely out of the bullpen at both locations and combined to pitch 71.2 innings, 42.2 of which came with Durham. At the Triple-A level he built a 3.80 ERA and 1.20 WHIP while fanning batters at a great rate of 12.2 K/9.