The defender was announced as our third summer signing on Monday, joining the club from Premier League side Leicester City.
Maguire wore the no.15 shirt during his two-year stint at the King Power Stadium, when the no.5 shirt was unavailable due to captain Wes Morgan occupying it.
Harry had previously worn no.5 at former clubs Hull City and Sheffield United, while he has tended to don the no.6 jersey for England.
The 26-year-old could be seen for the first time in his new United shirt during Sunday’s Premier League opener against Chelsea at Old Trafford.