Romelu Lukaku has a serious injury and he will stay out of the fields for months. He will not have a surgery but he cannot return before at least another month if not couple of months.
His team will have to deal with his absence for quite long time so unless they do very well in the first matches, they will be in hurry to find a suitable replacement
Lukaku's thigh injury likely requires a lengthy recovery and rest period. I've even read in the news that he could be out for 3-4 months. And I think that's quite a long time. So Napoli needs to act quickly to find a good replacement before the transfer window closes. And the transfer window is almost up, with Serie A starting this week.
There are reports that Napoli will be targeting Dovbyk, following Antonio Conte's request. However, we know Napoli aren't the only ones interested in Dovbyk, so we can be sure the competition for the Ukrainian striker will be quite fierce.
Napoli's first match of the season will be against Sassuolo, and we can probably expect them to win even without Lukaku. However, I'm concerned about the other matches against other strong teams, such as in Week 3, when Napoli will face Fiorentina. I wasn't worried about Napoli in the first and second weeks.