It seems quite clear that José Mourinho will be Manchester United's new manager next summer. According to Manchester Evening News, the Portuguese coach has let his inner circle know that he is ready to join United and Varane would be on his shortlist, per that same report.
Varane signed for Real Madrid during Mourinho's tenure. In fact, Mourinho gave him plenty of minutes in the 2012-2013 season, when Varane blossomed into the player he is now.
The French defender is still 22 years old and has four years left on his contract with Real Madrid, so any potential departure would be quite expensive for Manchester United. It seems that Real Madrid are still interested in signing De Gea away from United next summer, so Varane could be involved in the conversations between the clubs. He should be a crucial player in Real's plans and it would be surprising to see him walking away from the Bernabéu any time soon.