The Red Sox claimed Castillo off waivers from the Royals on Tuesday.
Castillo has collected a career 5.43 ERA, 1.37 WHIP and 47:24 K:BB in 59.2 innings over parts of two seasons in the majors, making six starts and 15 relief appearances over that span. It's not clear which role the Red Sox have planned for him. The 24-year-old does have an option year remaining, so Boston could stash him at Triple-A Worcester if Castillo doesn't earn an Opening Day roster spot.