From an ESPN Insider chat:
Rob Neyer: My guess is they'll keep Polanco, and Verlander makes his debut in September (unless somebody in the rotation gets hurt, in which case he could jump straight from Class AA to the majors). Verlander, for those of you who've not heard, is probably the No. 2 pitching prospect in the minors, behind only Felix Hernandez. And Verlander might have a slightly better shot of staying healthy, which might mean he's actually No. 1.
Now, we're all excited about Verlander, but calling him the first or second best pitching prospect in the game? That I hadn't heard yet, but I like it, I like it a lot.