I think Napoli's new signings have shown some good things, as Kevin De Bruyne has shown some good form in recent matches.
The news of Lukaku's injury has fans worried that their favorite club won't be able to compete for the title, and I don't think Lukaku's injury should take long to recover.
Inter Milan might take advantage of this opportunity to score a bigger goal early in the season.
I'm very sorry for Lukaku, he's a player I really like and appreciate for his mentality and his physicality, he's great and I think that without him Napoli could suffer and not a little, I also believe that they won't find a replacement easily, now they have to adapt more than ever
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 is true that Lukaku's injury will affect the team in the starting matches of the season and due to the fact that the nature of his injury has not been examined, it means that we don't know yet how long his injury will last but since it's a suspected thigh injury it might last a couple of weeks or months depending on the state of the injury. I think Lorenzo Lucca can perform as a striker, we saw how he performed during the preseason games so Napoli wouldn't need to worry much about Lukaku's injury and for the fact that Kevin De Bruyne is also very active in the midfield he can also assist is goal scoring and distributing quality passes that can lead to goal. I don't think Napoli needs to worry much about Lukaku's absence, they should just make use of the available strikers and see how they can have a good start till Lukaku returns from injury.