Castillo was recalled from Triple-A El Paso on Tuesday.
The 26-year-old hasn't pitched in the majors since 2019 due to numerous injuries but will join the Friars on Tuesday with Yu Darvish placed on the paternity list. Castillo has a 2.08 ERA, 1.23 WHIP and 45:14 K:BB across 34.2 innings with El Paso this year.