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Napoli will really be bad enough if they let Oshimen go. Because he contributed very well last season and scored a lot of scores, and became one of Napoli's key players in the Serie A victory.
Certainly it will have a certain impact, moreover they also have a new coach which of course will require more adaptation. However, with a coaching style that is quite similar to Spaletti's, I'm sure that Oshimen and the other players will find it easier to adapt and do their best like last season.
There are still a few more rumors of players who will be brought in by Napoli such as Max Kilman and others. So indeed Napoli certainly wants to defend the next Serie A title even though it feels a bit difficult.