What to expect from Ronaldo by Spalletti? I don't think Ronaldo will make much of an impact, maybe it will only add to Spalletti a new problem. His nature at Man United should know what will happen if he brings in Ronaldo.
I think it will be more trouble than profit. If Ronaldo's price is still expensive, of course it will not be worth the gain. I think Napoli are better looking for a talented young forward. It's not buying an old player who is starting to lose his best strength.
Ronaldo can bring more confidence for Napoli players. Also, italian league a bit have a slow playing rythm, so if Ronaldo join to Napoli, he can give a chance to bring trophy since with his skill and his performance, still applicable for Serie-A. Don't forget about money, merchandise sale can help their finance. The only problem is his salary, i'm sure maybe hard for naples to pay him.