Maguire is currently high on the list, costing the Red Devils £ 80 million when they signed him from Leicester this summer.
Napoli suffered a difficult campaign.
And owner Aurelio De Laurentiis blames Koulibaly for the players who rebel against him earlier in the season.
While United wants Koulibaly, club legend Andy Cole has urged his old team to conquer the Napoli star.
“I would be surprised if Manchester United spent it in another half-time – £ 85 million,” Cole told United Stand.