In my opinion, going with a 1-2-3-2-2 works best for Napoli with their current squad. Their defenders aren't that good, they need the numbers back there. However, 4-2-2-2 (4-2-4) works well as well. But you'd need two good full-backs and a good centre-back, because their defenders just don't cut it. To add to that, you'd need a midfielder to work with Hamsik, maybe throw in a winger.
So yeah, go 1-2-3-2-2 (3-3-2-2) to get going, and later transition the team to play 4-4-2, 4-2-4, 4-3-3 etc.