Sergio Reguilon has made 67 appearances for Tottenham since joining in September 2020
Reguilon was signed by United in August following injuries to the club's left-backs Luke Shaw and Tyrell Malacia.
The 27-year-old made 12 appearances, most recently as a late substitute at Nottingham Forest on Saturday.
He started on seven occasions, most recently in the 3-0 home defeat by Bournemouth on 9 December.
With Shaw and Malacia expected to return to action this month, United have opted to let Reguilon return to Premier League rivals Tottenham.
Spurs are fifth in the table, one point outside the top four, while Ange Postecoglou's side are also eight points and two places above Erik ten Hag's struggling United team with half the season played.
-- Courtesy of BBC Sport