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.
Madrid have spent big on Ferland Mendy in the left-back position but also see a future for Reguilon in the Spanish capital.
The youngster is a product of Madrid’s academy, emerging from their youth ranks to make his professional debut for the Castilla side in 2015.
He racked up 22 appearances for the first team in 2018/19, but Zidane decided a loan stint at Sevilla was the best move for his development last season.
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.
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“Luke Shaw I think is unfortunate in terms of his injuries. He’s played games but then he hasn’t been able to sustain a full season without any injuries, which is disappointing.
“But listen, if I was looking at it and looking at the team, in the forward areas I think one or two additions. One definitely, maybe two, are needed that can come in and affect that first team and push people, if not start.
“And then I think a centre-back. That area, for me, is where there’s a lack of real, genuine pace and I think the best teams around at the moment have a player in one of the two players that play [at centre-back] in a four that has genuine pace normally.”