According to a report from Football Insider, Everton are set to sell Mason Holgate in the summer in order to try and sign Koulibaly, who is Carlo Ancelotti's top target this summer.
Holgate agreed on a five-year deal with the Toffees last year when he was one of the best performing players at the club, attracting plenty of interest from across Europe.
However, Holgate has had a very mixed season this time, even though he has started regularly, and while Ancelotti admires Holgate's versatility, being able to play across the defence and even in midfield, he has not performed at the same levels of last season.
Holgate has made some crucial mistakes at the back, particularly against Tottenham Hotspur earlier on in the season, which have led to the defender facing plenty of pressure to perform better.
The 24-year-old defender currently has a market value of €18 million (transfermarkt.com), and with his recent contract, Everton will most probably want more for him in the summer.
With Gennaro Gattuso's time at the club still uncertain due to the race for the UEFA Champions League spots, it might be the perfect time for Koulibaly to move away from Italy after years of speculation.
Holgate has made 28 appearances in all competitions for Everton this season, with him struggling with an ankle injury at the start of the season.