Despite the heavy loss the Lions did finish the first half 2-1 ahead and Portugal U21 star Bruma in particular put on an impressive performance.
The 21-year-old winger put Galatasaray ahead on 39 minutes after he found the back of the net with a lovely finish after working a clever one-two passing combination with Sinan Gümüş.
Bruma joined the Istanbul giants from Sporting Lisbon two years ago but failed to settle in and was loaned out to Real Sociedad last season.
The young Portuguese winger is expected to stay at Galatasaray next season.
Gümüş scored Galatasaray’s other goal but the Lions were unable to replicated their first half display in the second half and conceded four times within 45 minutes.
United striker Zlatan Ibrahimovic ended up scoring on his unofficial debut inside four minutes while the other goals came from Mata, Fellaini and a Wayne Rooney brace.