Timothy Fosu-Mensah has completed his move to Bayer Leverkusen in what signals the first transfer exit from Old Trafford this window.
The 23-year-old heads to the Bundesliga side permanently in a deal believed to be worth in the region of £1.8million.
Fosu-Mensah had just six months remaining on his contract and was due to leave in the summer on a free.
United took up the option of extending his deal by a further year last summer, and have now cashed in on the defender, who joined them in 2014.
Ajax and Hertha Berlin were also reported to be chasing his signature.
A statement on United's website hailed a "fresh chapter" for Fosu-Mensah, and said: "everybody at the club would like to wish him the very best of luck with his future career."
He made 30 appearances in total for United's senior side, including featuring in three matches this season.
Ole Gunnar Solskjaer admitted last week that the versatile full-back had "not played enough" and was free to find a new club.
"It's difficult when you've got good players, talented players, and you can't give them game time," Solskjaer added.
Meanwhile Jesse Lingard could be the next United star out of the door, with French Ligue 1 side Nice keen on signing him.
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Both clubs are said to be in negotiations over a potential deal for the midfielder, who has only made three appearances this season.
United made the decision to trigger the one-year extension in Lingard's contract too, thus keeping him on the club's books until 2022, though he is still likely to move on before then.
Back-up goalkeeper Sergio Romero is another player who is expected to depart this month, with a return to Argentina on the cards with Boca Juniors, who have also held talks with Marcos Rojo.
