Castillo, who allowed two hits and a run in his spring training debut Wednesday, spent the offseason working on improving his slider, Bobby Nightengale of The Cincinnati Enquirer reports.

EDGE Analysis

The Reds want Castillo to improve his third pitch, whether it's his slider or it's throwing his curveball again, after that pitch disappeared from his arsenal last year. Last year Castillo only threw his slider 16.5% of the time, but had a 40.8% Whiff%, according to Baseball Savant. He was throwing it more often in August and September than he was earlier in the year, for what it's worth.