The 27-year-old joined from Spurs for the season on deadline day but the contract contained a break clause and United have chosen to activate it.
Reguilon made 12 appearances for United, the last of them as a late substitute at Nottingham Forest on Saturday, started seven games and is deemed to have made a positive impression at Old Trafford.
Shaw is out with a short-term niggle while Malacia, who is yet to play this season, is nearing a return. Diogo Dalot, who has sometimes been selected ahead of Reguilon, is seen as an option at left-back in the meantime.
In addition, Reguilon would have been ineligible for United’s next Premier League match as it is against Tottenham, his parent club, while they have fewer games in the second half of the season after exiting Europe and the Carabao Cup.
The former Real and Atletico Madrid defender is the second player to leave United in January, with Donny van de Beek joining Eintracht Frankfurt on loan for the rest of the campaign.
Reguilon’s return could afford him a chance at left-back with two of Ange Postecoglou’s left-backs, Ryan Sessegnon and Ivan Perisic, sidelined and a third, Ben Davies, playing centre-back because of injuries in the middle of the defence.