If Napoli doesn't have a second striker that can fill in Lukaku's space, I agree with you that Napoli could struggle to win their matches, but if Napoli has a backup striker they can still make use of until Lukaku returns from injury, the striker can help Napoli win their matches this season. If Napoli feels they need a new striker since Lukaku is injured, the transfer window is still open for them to bring in another striker to start their Serie A campaign.
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.