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.
The issue of a second striker is one issue that could have cost Napoli the title last season, Antonio Conte's reliance on a single striker for the entire season is a wrong tactics because it would wear out the striker due to the excessive work rate that the striker has to face all through the season, Romelu Lukaku is a strong player but his health has to be put into consideration and that's why another striker has to be brought into Napoli.