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It's been reported since the last transfer window though that nkunku willing to move to premier league side like chelsea and manchester united but the club stated that going to reject any proposal coming to them but this time the rumor seems a bit different.
Chelsea might have an insider agent that could make this transfer happened despite the long term contract.
There is many deals that failed for both Chelsea and ManU so it is expected to see them trying to consolidate the squad with good players. The situation in Chelsea is quite more stressing with many weird results..
The fact that a player increased his contract send 2 messages: whether you forgot about the transfer or you pay more! In this situation Chelsea will pay more imo to get him.