Reguilon looks set to leave the Santiago Bernabeu this summer, as Ferland Mendy and Marcelo are ahead of him in the pecking order under Zinedine Zidane.
The 23-year-old is keen on a move to Old Trafford, as he has been assured of a starting place.
Having played a crucial role on loan at Sevilla for the past season, United boss Ole Gunnar Solskjaer hopes Reguilon will strengthen United’s left-back position.
Last season, the position was occupied by Luke Shaw and 20-year-old academy product, Brandon Williams.
Spain Under 21 international is likely to cost United around £25m, with Real Madrid adding a buy-back clause in the contract.