United made Harry Maguire the most expensive defender of all time when they signed him from Leicester for £80m at the start of August.
And the Red Devils attempted to pair the England international with Kouliably in the heart of their defence.
United submitted an offer of £82m for the Senegalese stopper before England's transfer window closed last week.
But Napoli, who are desperate to keep hold of their leading defender, turned down the bid.
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