You have to remember that they have also bought Lucca as well.
https://www.transfermarkt.com.tr/lorenzo-lucca/profil/spieler/572265This dude is not nothing, he is on loan of course but he is a great player and he could take some of the games. I do not see them playing with two strikers, that is not the way Conte plays, so that means Hojlund and Lucca will end up battling for the position.
Hojlund is better in my eyes, while he wasn't great at United, he was awesome in Italy before he went there, so he could go back to being awesome here again. If that does happen, then he can cut Lucca and play. If not, and Hojlund plays a few games not so well, Lucca is right there to take his spot.
Plus, training is important for them now as well, because if they do train then we are going to see these players battle there, they have to take training seriously.
I don't think Conte would be convinced about him yet. Therefore I wouldn't be surprised if Højlund is given a chance in the starting lineup right away.
Statistically they haven't been very different players. Both of them aren't that impressive. But who knows, maybe Højlund will prove most people wrong about him at Napoli. I would love to see it. He is still young.