Although the Red Devils are not short of options in the middle of their defence, they have struggled this season with goalkeeper David de Gea making the most saves in the English top flight.
Although many players have performed on average, United captain Harry Maguire has been a target for criticism, and it remains uncertain how much longer the England international will remain as first choice.
According to Fichajes, the Mancunian giants may make a new bid for Koulibaly, who has been heavily linked with a move to Old Trafford in the past.
The centre-back is contracted to Napoli until 2023.
The report claims that the Senegal international may be available for €40m, a cut-price fee which may alert a wide range of clubs including Manchester United.