Castillo is an interesting case. On one hand, a lot of his numbers are kind of what they've been. His K% is within around 1% of where it was last year, he's not walking guys, his Stuff+ is the same, his arm angle is the same, the velo is fine, he's not giving up a lot of HR's and the LOB% is low for what it probably should be.
But then you look at the whole and it's just not working this year. The fastball last lost some shape, which you can see in Stuff+ and it's movement profile on Savant, his K% was down below average last year. He's getting less chase and his savant page last year wasn't sparkling. So is it just the hens coming to roost?
With that said, I'm mostly "out" on Castillo. If you can bring the K% up to where it was before last year, or there's some mechanical thing the Cubs really believe in he could be a cheap, controlled SP. But I'd rather just pay up for someone better, Or even a rental.