Marcus Rashford came off the bench to score a clinical hat-trick as Manchester United continued their recent momentum by taking apart RB Leipzig in the Champions League.
England international Rashford came on just after the hour mark and netted three goals in 16 minutes, first running clear to calmly slot in, then smashing in a terrific second, before drilling home a third in stoppage time.
Teenage forward Mason Greenwood scored his first goal in the competition, latching onto Paul Pogba's defence-splitting pass before drilling a precise, low finish into the far corner.
Greenwood produced another thumping effort straight at Peter Gulasci, while the visiting goalkeeper also kept out Anthony Martial's poked effort.
But when Martial was tripped in the box with three minutes to go, he converted the penalty to score his first goal of the season.
Ole Gunnar Solskjaer's side backed up their excellent win at Paris St-Germain in their opening game by collecting another impressive three points to extend their unbeaten run to four games.
United's backline had a comfortable night, David de Gea keeping out efforts from Christopher Nkunku and Ibrahima Konate.
In the Group H's other game, last year's runners-up Paris St-Germain picked up their first points with a 2-0 win at Istanbul Basaksehir.
Rashford can do no wrong
United had said before the game that they were "bemused" by the government's decision not to allow fans into Old Trafford despite making the stadium a socially-distanced venue for 23,500 supporters.
And with United playing their first Champions League game at home since losing to Barcelona in the quarter-final in April 2019, it was a shame the supporters missed out on their side's best performance of the campaign.
Solskjaer's side were up against the current Bundesliga leaders and a team that reached the semi-finals last term, but made the German side look ordinary with a superb, all-round performance.
United's dominant display was highlighted by the fact that Leipzig had won six and drawn one of their seven games this season, conceding just three goals in the process.
Man of the moment Rashford again made an impact, netting with all three of his shots on target.
Honoured with an MBE earlier in the month and continuing his fight against the government over free meals for school children off the pitch, Rashford showcased his prowess on it with a first treble in Europe's elite club competition.
His first was initially ruled out by the assistant referee for offside, but replays showed he had timed his run to perfection from behind the halfway line, bursting clear from Bruno Fernandes' raking pass and opening up his body to before slotting home.
The second came with a run to the edge of the box before powering his finish into the corner, while he completed his hat-trick from close range.
United on a run
Since the 6-1 humiliation at the hands of Tottenham, United have collected three victories and a draw, including back-to-back clean sheets.
The much-maligned defensive duo of Harry Maguire and Victor Lindelof kept the Leipzig forward line quiet as they secured another welcome shut out.
In front of them, Netherlands international Donny van de Beek was given his third start, after two in the Carabao Cup, and the midfielder was steady and tidy, completing 89% of his 28 passes and winning the ball back six times in his 67 minutes on the pitch.
Such is United's depth, star man Bruno Fernandes was only on the bench, but came off it to grab another assist.
Few would have expected the Premier League side to claim six points from six against supposedly the two most difficult opponents in the group.
But they have set themselves up perfectly to reach the knockout stage, with back-to-back games coming up against the pointless Istanbul Basaksehir.
United are at Old Trafford again on Sunday, when they face Arsenal in the Premier League (kick-off 16:30 GMT).
Super sub Rashford - the stats
Manchester United's victory was Ole Gunnar Solskjær's 50th match as a manager in Uefa competition, with the Norwegian overseeing 11 wins in 18 matches for Manchester United (61% win rate).
Marcus Rashford is only the second Manchester United player to score a hat-trick as a substitute after his manager Ole Gunnar Solskjaer did so versus Nottingham Forest in February 1999 in the Premier League.
Manchester United have kicked off a Champions League campaign with back-to-back wins for only the second time since Sir Alex Ferguson left the club in 2013, also doing so under Jose Mourinho in 2017-18.
RB Leipzig's 5-0 defeat is their heaviest loss in their short European history, whilst the scoreline represents Manchester United's biggest victory in the Champions League since November 2013 v Bayer Leverkusen (5-0).
Despite only making his debut in February 2020, no Manchester United player has registered more assists in all competitions than Bruno Fernandes (12) under Ole Gunnar Solskjær, despite the Norwegian taking charge in December 2018.
Only Brian Kidd (18y 275d) and Wayne Rooney (18y 340d) have scored a European Cup/Champions League goal for Manchester United at a younger age than Mason Greenwood (19y 27d).
Manchester United striker Greenwood has scored six goals goal in major European competitions as a teenager, the joint-most of any Englishman alongside team-mate Marcus Rashford.
Manchester United striker Rashford became just the fifth player in Champions League history to score a hat-trick as a substitute, though his 27-minute cameo is the fewest number of minutes played while netting a treble in the competition.