Fabian Ruiz is a talent but also already very good - He is a better player for now as well...
Allan also is stronger and definitely Koulibally..
Koulibaly is not having a good season to these standards. Maksimovic and Manolas can replace him just fine. Fabian is very good but not key to Napoli yet. They can play with Zielinski or Elmas there to little change. Allan is much better in midfield at the moment, he is the only one with a shout.
In a Napoli match it's clear who is running their game. From the wide left this season (worse position for him) Insigne has the touches of a midfielder and the shots of a striker. It's not just that he leads them in G+A but also in their flow of play.
Fabian, Lozano, Milik, Zielinski, Elmas... the younger players they have look great but aren't yet their key players. The older team is still better, which is normal as they are the finished product, the transition hasn't occurred yet. Mertens, Insigne, Allan, Callejon and Koulibaly are still the core.
Give Fabian time and he will dominate.