Athletic guys can play right out of the gate.
Tell that to JJJ and E'Twaun- they were two of the most athletic players on the 2011-12 Celtics and they hardly saw the court. Athleticism definitely makes the transition to the NBA game a little easier, but I would argue that maturity, willingness to be coached, bball IQ, and countless other traits are equally important for a rookie's immediate impact.
We'll see if Melo can make a contribution this year- I don't know how so many people are already so sure of his abilities/potential after a few summer league games. He IS going to get better, that much is obvious. The questions that remain are: How good will he be? How long will it take to get there?
The answers won't be known for at least a few years-this coming season will give us some hints but we still won't see anywhere near his full potential. Even if he doesn't play a single minute this year picking him will have been far from a waste- he is a PROJECT and projects take time to develop.
I'll have to read more about this Rob Mac guy, it sounds like he knows what he's doing. Hopefully he sticks with Fab, and maybe even develops some sort of relationship with the Celtics organization.