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.