Lukaku has been sidelined for a long time, and it's estimated he'll be sidelined for at least three months. This could cause irreversible damage for a player his age. He might not be playing as well as he is now when he returns in three months. Therefore, it would be in Napoli's best interest to replace the Belgian striker before the transfer window closes. Napoli's chances of winning the Champions League and the Italian league this year are quite low, but they need a good striker to at least win the Italian league.
I agree with this, a player that age suffering an injury that will take him this long to fix can literally cause him a lot, recovery may not be 100% such that his efficiency matching up with his status quo, at this point seeking an alternative is the best option they have to explore because even if they have to wait for him to recover, in the end it may not worth the wait because even after recovery he will need probably a month to get back in shape completely to deliver as expected of him of which the season must have gotten close to mid season by then leaving the team with almost no time on their side for the season.
It's time for Napoli to find a suitable replacement for Lukaku to fill the void in their forward line. I'd even say Napoli shouldn't expect anything from Lukaku this season.
Lukaku has a serious injury that is expected to take three months to heal, and there's no guarantee he'll be able to return to top form after recovering from it. Now is the time for Napoli to race against time before the transfer window closes to find a new striker.