Napoli needs a supporting striker to help Lukaku in the team I know that Lukaku is the best man for Conte the manager but I see the team struggling to win matches in his absence because the team is gradually depending on him to convert most of the goals. The transfer window is still open but may be the manager wants to still believe his squad to deliver a good performance and score goals without Lukaku that's why he's not making moves to sign another striker.
Last session Napoli did it without a supporting striker for lukaku and so it looked like they can always do it but he is human and will have some lapses at some point even in his presence there are times the team does struggle for a win because he will not be in his absolute form everyday, with time it become very clear the need of another striker for Napoli, maybe at that time they will put it in to consideration.