Just different usage rates for the rest of the players. The hardest thing to replace would probably be his ability to get to the free throw line, but Powell has a pretty good mentality in that he drives to the rim a lot.
Casey is not going anywhere. Masai already said this before the series. It's not happening, get it out of your head, stop being happy that they're losing FFS.
I'm not sure on the numbers regarding his spin rate, but Smyly is very similar to Estrada in the way that he goes about pitching. He just happens to get more strikeouts as well.