Napoli messed up by spending big on Manolas and not getting a quality midfielder to replace Hamsik.
Koulibaly always felt a bit clumsy when he's not next to a calming presence and Manolas is the definition of a clumsy defender. He's making Koulibaly seem worse and their defense is going to be in shambles as Meret is just a kid and their fullbacks aren't really great defensively.
They are paying for their bad judgement now but anyone could have seen that coming.
Their best bet was to try and get Miranda from us and go big for someone like Lo Celso, Praet or if they had the funds, Rakitic.