It is actually depending on the board of Napoli itself. If they want to lower the transfer fees of Osimhen, it won't be so difficult to sell Osimhen. PSG agrees to sign Osimhen at €120 million, it is a huge money.
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I don't think Napoli are ready to lose their keyman to any elite club this summer, they've solid plans for the Nigeria international and keeping him would be their absolute decision. On the other end, Paris Saint-Germain are very persuasive in this case, since losing one of their keyman in the frontline, which will definitely affects them in all angles next season. It's up for Les Parisiens president, Nasser Al-Khelaifi to play his role and also involved the current coach, Luis Enrique to cooperate and tangled good signings.
Based on the news circulating, Osimhen was not included in the Napoli squad for the trial match or friendly match and as such, I think Conte really wants Lukaku rather than Osimhen. Thus, of course Napoli will still to sell Osimhen in the transfer window at this moment but indeed, because lastseason Napoli also extended Osimhen contract and I think Napoli will keep him for the long term.
But what is happening now is that Napoli and Conte also have different plans for next season. Because after all,if they have the same plan then of course it is very possible for Osimhen to be retained, because Napoli also still needs him. But yes, there is nothing that can be done if the coach really wants Lukaku and thus, it is very possible for Osimhen to leave.