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.
It seems Napoli should act quickly to consider this before the new season begins, because if Lukaku's injury still hasn't recovered by the start of the season, finding a new striker would certainly be a wise move for Napoli. However, they also need to ensure that the new striker is good, even if not quite on par with Lukaku's achievements. If the new striker is just a mediocre player, it certainly won't be enough to help Napoli start the new season and Napoli could struggle to win consistently this coming season.
Napoli shouldn't delay in trying to fill his void. If he's facing a long-term injury, a transfer must be made in that area. Romelu Lukaku is a truly important player for Napoli and a crucial figure for the team. His replacement needs to be at least as good as his quality and be comfortable with being on the bench, as he'll likely be the one to return to the starting lineup once Lukaku recovers.