As much as I'd like to see Castro take this, I don't think he'll come close to getting enough votes. Heyward seems to have been anointed the RotY since day one, and his numbers are pretty decent (.277 BA, .387 OBP, .862 OPS, 16 HR, 64 RBI, 73 Runs, 9 SB). Those are pretty great numbers. If Posey had a full season at his current rates it would be him hands down, but the lower game total and the strong competition probably pushes him out. Garcia will get some votes, but I don't think he'll get it.
I'd say the only way for Castro to make a real run would be to win the batting title. That's not likely, but not crazy either as he's currently in fifth place in the NL.
All in all, it's nice to see an injection of strong young talent in baseball.