I think Nova is ready for the #5 starter spot. He has strikeout ability, but also great stuff to pitch to contact to. He just needs to work on holding runners and poise, which is something you dont do very much in the minors when nobody seems to get on against you and the pressure is much less. But we would need to build in some serious contingency plans.
Let's say Andy retires and Lee goes back to Texas. I would assume the Yankees would go back and get Chien Ming Wang. He's finally healthy, and even if he only tops out at 90mph, that sinker could get outs. Then I agree, I would look at Webb. The only problem is, Webb was topping out at 85mph in his spring workouts, but that was pretty fresh out of his rehab. If he regains some arm strength, you should see his sinker go back into the high 80s range, which could make him useful. Then you have to think about innings eaters like Kevin Millwood or Vicente Padilla. Regardless, slim pickings after Lee, which makes him pretty necessary. You also have to think about moving Joba back to the rotation