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When signing a replacement player, clubs consider the player's recovery time. If Lukaku is out for three months, they'll definitely sign a replacement, but I have no information on this. I don't know if it'll be a long or short-term injury. If he's out for just a week or two, it wouldn't make sense for Napoli to spend a lot of money to sign a new player. However, if the injury is long enough to affect the Champions League and Italian League, they absolutely must have a good replacement.
If I'm not mistaken some time ago, Napoli were also quite interested in being able to bring Garnacho from Manchteser United, but until now there has been no continuation regarding the transfer of these players, whether Napoli abandoned their intention to be able to recruit the Argentine player. And I think Napoli have strong enough finances to be able to bring a new, better striker into the team, especially after they get a big profit from the sale of players in the transfer market this summer.
Yes, there may be major changes mid-season, but with a tight schedule, the possibility of Inter Milan players getting injured is always there, especially since they are filled with older players who are quite vulnerable to injury. A tight schedule is always a problem for every big team, and that happened to Juventus and AC Milan last season, so if Inter Milan doesn't prepare thoroughly, they might experience what Juventus and AC Milan experienced last season.
Inter Milan knows how to maintain the performance of its players, but of course even so the possibility for its players to get injured is always there, especially when faced with a fairly busy match schedule. But we will see what they will do to face this season.