According to Corriere dello Sport (via the Metro), United are finalising preparations and will bid for Koulibaly soon.
Ole Gunnar Solskjaer is reportedly desperate to sign a top centre-back to bolster a leaky defence.
United's proposed bid would be £84 million, which would make Koulibaly the most expensive defender in history.
However, with Koulibaly's release clause sitting at £133 million, Napoli are thought to be hanging on for a fee closer to that figure.
If the Reds fail in their pursuit, they will focus on other targets such as Ajax's Matthijs de Ligt and Leicester's Harry Maguire.
READ MORE...
Manchester United's failure to hire a director of football reveals the indecision gripping the club
What success – or failure – in the Nations League could mean for England
'I have found myself again. At Manchester United, I continued working mentally and physically - I was preparing for this moment' United star provides insight to difficulties at Old Trafford
'When I came to Madrid, I thought that I was Messi and Cristiano in one body – I was a punk, a nobody': Real Madrid star reflects brutally on his younger self
