Romelu Lukaku will return to the pitch in December. This means he won't be able to play for Napoli anytime soon. That's why the Hojlund transfer was made. In his absence, the Dane will perform at his best and will likely come on as a substitute until Romelu Lukaku regains some playing rhythm after returning from injury. In the meantime, the Danish striker could be considered. Perhaps what you're suggesting will come true once Romelu Lukaku returns to the pitch.
Lukaku will not mind too much about this season with Napoli as he is old and his chance of playing for Belgium national team in big tournaments like Euro and World Cup will be very limited. He will care more about chances of being able to play in the World Cup 2026 next summer rather than how many appearances he will play for Napoli in this season.
He needs good recovery from the injury first, then needs appearances with Napoli squad in order to get as best fitness and performance quality as possible as important preparation for World Cup 2026. This means it's hard for Lukaku to be called for World Cup 2026 if his performances for Napoli are too limited and bad in quality.
Currently, Hojlund has officially become a Napoli player, registered in Serie A since 01.09.2025. And according to the latest news, he will be included in the UCL squad, and could be in the lineup for the next Serie A match against Fiorentina. And as you said, it's the first week, and we can only hope for the best possible outcome, but we can't force him to perform that well straight away. But if he can do it, it will be a plus point that will have a big impact on him.
Napoli consider Hojlund as a replacement for Lukaku but unfortunately this young player will not be able for playing instantly for Napoli. They will have to give Hojlund a little bit of time while with his past experience in Serie A in Fiorentina skirts, it seems Hojlund won't have trouble to adapt to Serie A again.
Napoli played well in first two matches in Serie A but they truly need both Lukaku and Hojlund in remaining matches of season in order to defend Scudetto successfully.