In the first transfer window, I tried to invest in players Napoli need. I would definitely go for a backup/first team midfielder. I chose Xhaka, and switched to a 4-3-1-2 because I wanted Jorginho to be involved. I know they would propably never play this formation in real life, but it is working for me, and I can always switch to 4-3-3 in-game. Other than Xhaka, I invested in talent, and I got rid of the dead wood, and players that I don't like. I sold De Guzman, Mertens (don't like him in real life, and I don't like to play with players I don't like) and some others.  
I bought a back-up and soon first-team young-ish CB in Dragovic. I also bought Barba and Benassi (real-life targets).