The Reds top Group L with 10 points and are already safely in the December 16 draw for the round of 32 with two matches to spare.
Mason Greenwood, who hit the winner in the opening group match at home to Astana in September for his first senior goal, opened the scoring against the Serbian side with a clinical 21st-minute strike.
Anthony Martial bagged a sizzling solo goal in the 33rd minute when he shrugged off a string of Partizan challenges to score and Marcus Rashford struck a blistering third in the second half to complete United’s comfortable evening.