I have a feeling at least one of them will open the season in the rotation out of spring training next year. While I don't think neither of them will be quite "ready", I also don't think either is too far away that opening the season would be too big of a stretch.
My prediction for the Tigers' rotation is like this:
1)Bonderman definitely returns
2)Robertson almost definitely returns
3)Tigers will sign a free agent. Will probably pursue Burnett, but not get him and end up overpaying a Millwood-type
4)Maroth has about a 50/50 chance of being traded. This is where my uncertainty is. If he's traded, Ramon Colon could be in this spot, as Dombrowski is very high on him. Either way, if only one of Verlander/Zumaya open the season in the rotation, the guy in this spot will be looking over his shoulder.
5)Zumaya or Verlander
I'd have given the edge to Verlander with the way they've given him spot starts this year, but he's missed a fair amount of time now with a tired arm, so I'm guessing if it comes down to one of them, the winner will be whoever looks best in spring training.
The above is my opinion. This organization historically has done some crazy things, so it's hard to be accurate with these sort of predictions.