The Morocco international joined the Reds on Tuesday, in a deal subject to registration that will keep him at Old Trafford until June 2028, with the option of an extra year.
The former Ajax and Bayern full-back will sport the no.3 for his new side, which has been a famous figure for United down the years.
As the traditional number for left-backs in England, the likes of Tony Dunne and Arthur Albiston were long-time wearers, while Denis Irwin was the first player to be listed at no.3 when the Premier League mandated squad numbers from the 1993/94 season onwards.