According to Duncan Castles of The Sunday Times, the Senegal international is the nidseason priority target for Red Devils manager Jose Mourinho, as he seeks to improve the team's leaky defence.
However, it's reported United would have to pay a world-record fee for a defender if they want to prise the player from the Stadio San Paolo.
"Napoli owner Aurelio De Laurentiis has made it clear that he considers Koulibaly central to a squad that lies second in the Italian championship and leads Champions League Group C ahead of Tuesday’s tie at Anfield," Castles said. "While De Laurentiis may prove easier to deal with should Napoli fail to reach the knockout stages, the club is under no great financial pressure."
Following Dec. 1's draw against Southampton in the Premier League, Mourinho was asked directly about the chances of signing Koulibaly:
Castles also reported United officials have already held talks with the Serie A side with a view to signing Koulibaly in January and were quoted £90 million.
For the Red Devils, bringing in a defender of the Napoli man's stature would be an enormous coup, as he's developed into one of the best centre-backs in the world over the past couple of seasons.
Defensively Koulibaly is a colossus. Rarely is he beaten in physical battles because of his physique, strength and speed, while during his time in Italy, there's been a clear improvement in his defensive positioning and reading of the game too.
He rarely gets flustered on the ball, making him the complete package when it comes to playing at centre-back:
The traits Koulibaly possesses are at odds with the way United have defended for long spells in the 2018-19 season.
Mourinho doesn't appear to have settled on a preferred centre-back duo because of fluctuations in form and a number of injury issues. Those who have played haven't taken their opportunities, either, with opponents finding United far too easy to cut through.
Ahead of United's 4-1 win over Fulham in the Premier League on Saturday, football writer Daniel Storey put the team's abysmal defensive record into perspective:
Koulibaly's addition would go a long way to alleviating those issues, as he's a leader at the back and good enough to build a defence around. However, it doesn't appear Napoli are willing to do business for anything other than an extraordinary amount of money.
With United down in sixth place in the Premier League and a long way short of the five sides above them, you would also have to question whether someone of Koulibaly's stature would be keen to make the move to Old Trafford during the peak years of his career.