But current Bernabeu star Marcelo is the reason the deal is being held up at present.
The Brazilian is 32 and will be reaching the end of his Madrid tenure in the coming years – and as such, the Spanish side want to ensure they have a buyback clause in any Reguilon exit negotiated, according to El Espanol.
Madrid want the option to buy the 23-year-old back in the next 24 months if he moves to Old Trafford and flourishes.
United, however, want a straight deal with no added extras or clauses involved.
Spurs and Sevilla both wanted to sign Reguilon this summer but appear to have lost out to the Red Devils, who are closing in on a move.
United have already signed Donny van de Beek in recent weeks.
But club hero Rio Ferdinand wants to see a few more additions brought in between now and October 5.
“I would say definitely after Van de Beek they need probably – if they’re going to really compete – three or four [signings]. Players that can go in and actually affect the first team, the first XI,” Ferdinand told BBC Radio 5 Live.
“And Reguilon we’re talking about, the lad at Sevilla who’s from Real Madrid, he was voted the best left-back in La Liga so really good credentials…