Henrikh Mkhitaryan scored a goal of the season contender as United beat David Moyes struggling Sunderland for a fifth successive win.
The Reds were dominant throughout but only a Daley Blind strike - his first of the season - separated the sides at the interval.
Zlatan Ibrahimovic continued his rich vein of scoring form and Mkhitaryan's acrobatic finish made sure of the win despite Fabio Borini's late consolation.
The victory moved United level on points with fifth-placed Tottenham ahead of the North London side's trip to Southampton on Wednesday.
Despite the Reds domination throughout, the third-from-bottom Black Cats almost found themselves against the run of play ahead after 18 minutes.
Jermain Defoe latched on to Jordan Pickford's long clearance and lashed a shot wide before Patrick van Aanholt forced an excellent save from David de Gea with a 20 yarder.
Paul Pogba went close shortly before the half hour when a slick one-two with Jesse Lingard on the edge of the box saw his strike from distance rebound off a post.
Juan Mata was barged to the ground by Lamine Kone and De Gea was then called into action twice in quick succession at the other end.
The stopped saved well from Victor Anichebe before he beat away Borini's effort as the visitors grew in confidence and belief.
United found the vital breakthrough six minutes shy of the interval when recalled left-back Blind latched on to Ibrahimovic's through ball and finished well from an acute angle.
Man of the match Pogba, so impressive in his all-round play, almost doubled the lead before Ibrahimovic fired over the bar having been released by Mata.
Mkhitaryan fizzed an effort narrowly wide shortly after his return to the fray following injury and Pickford saved from Ibra as United pressed for that elusive second goal.
It duly arrived eight minutes from the end when Pogba picked out the Swede and he finished coolly for his eleventh goal in ten games.
Ibrahimovic then turned provider when he found Mkhitaryan who capped a brilliant Boxing Day performance with a stunning back-heeled finish that will surely be one of the front runners for the club's goal of the season award.
Former Liverpool man Borini volleyed in a late consolation for the relegation-threatened visitors but David Moyes was well beaten on his first return to Old Trafford.
Five in a row!
Overall team performance: 7/10
United Faithful Man of the Match: Paul Pogba.