"I think he got to where he was a two-pitch guy, and this is a guy that has four pitches. He's had some ups and downs. There's been some inconsistency, he's walked more people than he's accustomed to walking. It all is part of a learning process for him. We're trying to get him back to using all his pitches."
So what does that mean? Nova's repertoire is dominated by fastballs and curveballs, but he has thrown at least one changeup in each of his starts this year. Anyway, Joe mentioned four pitches, which would imply that he sees the slider as missing in his repertoire. I would agree wholeheartedly with Joe's suggestion; for a guy who's struggling to miss bats, it can't hurt to try another breaking pitch.
UPDATE, 6/2: In a George King column, he says that Nova "vowed to use his slider tomorrow night with a fastball and curve." I'll be on the lookout.