The Red Devils are preparing an opening offer for the 23-year-old, according to Ed Aarons and Fabrizio Romano of The Guardian. Completing a move for Reguilon could be complicated, as the Premier League club is reluctant to match Real Madrid's €30-million price tag.
United are likely to submit an offer below Madrid's valuation, and they'll reject the Spanish side's attempt to include a buyback clause in any potential deal, Aarons and Romano add.
Despite playing a starring role last season while on loan at Sevilla, Reguilon isn't expected to be in manager Zinedine Zidane's plans for the 2020-21 campaign.
With that in mind, the Spanish defender, who has three years left on his Real Madrid contract, is understood to be hopeful of joining Manchester, where he would compete for playing time alongside left-back Luke Shaw.
Last season, Reguilon appeared in 38 matches in all competitions, which included a starting spot in Sevilla's lineup for their Europa League final win last month.