If Napoli is actually interested in defending the title successfully, they should actually think about bringing in players who will make the team strong. Defenders are obviously going to be very important for the team. Losing a really good player, like Min Jae Kim is obviously going to be big for a club that is defending the title.
After selling a key player like Min Jae Kim, I believe they knew what they were getting into. Napoli is not such a small team that one would assume they only desire to make revenue by developing players and selling them to other elite teams for profit. I believe they should have prepared for this by either bringing in a new player to replace him or making changes by creating a powerful squad from the existing players.
My recommendation is that Napoli seriously contemplate the recruitment of a formidable player - someone capable of seamlessly filling the void left by Kim. However, it appears that thus far, the club is attempting to address this issue with their existing squad members. The predicament lies in the fact that the current players may not possess the necessary qualities to strike a delicate balance within the team.
That is not exactly true for me. One player in a team is crucial checking from the void he fills in the squad. However, making alterations to the squad after only one player leaves will not be a major issue for the team. A good and experienced manager is required to carry this out for the team.