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.
After yesterday's failure to bring in Darwin Nunez, there are still no rumors that Napoli will recruit a new striker, but of course with Lukaku's injury, this forces Napoli to immediately recruit a new striker. But on the other hand, I also don't know how long my injury will take to recover, if it's only in a week or two, maybe this won't be a problem, the arena that Napoli will face in the first and second week is not a difficult opponent, where they will face Sossuolo and Cagliari. But if it takes longer than that to recover, then Napoli really needs to immediately bring in a new striker who can replace Lukaku's position temporarily.