Knicks could have got him for cheaper contract, as he would have been guaranteed with the limited role in NY (probably supporting Melo and co. for their stat building), while he is capable of growing further.
I'm pretty sure Fields will have a chance to score 10+PPG (with sweet passes from Lowry/Jose) with possibly 7+ RPG (with Bargnani
on the court).
The only concern is whether he could defend those top class SFs, like LBJ, KD, Melo, PP, etc. If he can effectively defend them, then Field's 20M/3yr contract is a crazy good deal