Castillo spent a portion of spring training working on improving his slider, C. Trent Rosecrans of The Athletic reports. Pitching coach Derek Johnson spoke of that work developing the slider: "There were some (strikeouts) on his slider, which I think has gotten better. I don't have the numbers in front of me on what he struck out guys with. Of course, the changeup is a weapon. I also think in time guys are going to start looking for his changeup more."
Last year, Castillo used his slider the least often of his four pitches, throwing it 17 percent of the time. It generated a lot of whiffs (47.5%) and a low batting average against (.190), but opposing hitters did some damage when they made contact, slugging .452 against it. The changeup will likely always be Castillo's best pitch, but he would benefit from a second reliable offspeed offering.