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.
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.
Most times it's very difficult to replace a player that is very good. It's not like that player is the best in the world in his position, but getting a player better than or as good as the player that left can cost more that you imagined.
Imagine Napoli goes all out of a quality defender that is up to the pedigree of Kim, how much do you think they'll spend? In my opinion, it would be nothing less than €70 million. This is a club that sold Kim because they needed money to balance some things.
When a player like that leaves, it's not a must to get an equal replacement, the team can adapt and play without him and it will be like he never left.
Napoli have not sold many of their top players which is a good thing. With the players they have they can compete for tye league next season. They just need a few additions for quality squad depth and they'll be good.