Both Liverpool and United have both been linked with a move for the centre-back despite the coronavirus pandemic likely to reduce spending in the next transfer window.
At the beginning of May, Gazzetta dello Sport claimed that Liverpool are 'on pole' for Koulibaly, ahead of arch-rivals Man United, with their 'prestige' giving them the advantage over other sides.
The Senegal international is among the most highly rated centre-backs in Europe, following his stellar performances in the Serie A and Champions League in recent seasons.
One report claimed that football's recent shutdown would see Koulibaly's fee drop from around €100 million to €75 million.
According to unexperienced Giuntoli, however, Napoli are in no mood to lower their valuation.
"Kalidou is a great player, who still has a long contract with us," the director said (via the Daily Mail).