The 25-year-old is believed to be keen on a move away from the Bernabeu and has reportedly told the club that he wants to seek a new challenge this summer.
Madrid pipped United to Varanes signature in 2011 after Zinedine Zidanes intervention but the Red Devils have never given up hope of signing the former Lens defender.
Ed Woodward vetoed moves for Jose Mourinho targets Jerome Boateng and Diego Godin last summer as he wanted a long-term solution to the clubs defensive problems.
United are willing to pay around £100m for him this summer and, though Madrid are considering a new contract renewal for the defender, theyre also drawing up contingency plans for if he leaves.
Koulibaly, 27, had been a target for United under Mourinho but the Senegalese defender opted to sign a new deal with Napoli.
However, he would likely jump at the chance to move to Madrid and AS claim Zidane has identified him as a replacement for Varane.
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