The Reds Devils trio Mason Greenwood and France’s Anthony Marciall and Marcus Rashford scored respectively in the 22nd, 33rd and 49th minutes.
Earlier Thursday, in the same group, Dutch Alkmaar swept the hosts, FC Astana Kazakhstan, with five goals.
With this result, Manchester United led the group with 10 points and qualified for the second round, while Alkmaar came in second with 8 points, Partizan third with 4 points, and Astana in fourth and last place. without any point.
Man United are in a major failure of the Premier League, having suffered four defeats in 11 rounds so far to fall to 10th place with 13 points.