His prospects for success/failure will come down to what kind of coach he is (X&Os, teaching, strategy). Listening to him speak, he appears to have a much better grasp of the technical and strategic aspects of the game than Lavin already. Even if Mullin turns out to be a great X&Os guy, he will need an assistant who is competent in that area with recent game planning and sideline experience this first season, in order to help him prepare for the daily structure and preparation of it all. It shouldn't take more than about 1- 1/2 seasons of observing his team to know whether or not Mullin has the goods as a strategist.
He has no experience but has a high b-ball IQ and determination. Was a bit of a risky move but if the guy turns out to be a great coach (and I wouldn't bet against him), it will mean great things given his stature in the City.