Last night, for the second time in three months, they put six goals past the Danish champions in Denmark's national stadium.
Last time was a pre-season friendly, this time it was for real. Taken together with Bayern Munich's 1-0 win over Barcelona, the consequence is that United this morning stand at the top of Champions' League Group D and with a far better chance of qualifying for the knock-out stages than once seemed likely.
Ahead after three minutes through Ryan Giggs, United ensured they would not let slip their advantage as they had done against Bayern and Barcelona by scoring five more goals in the next hour. Giggs scored again then Andy Cole, Roy Keane, Dwight Yorke and Ole Gunnar Solskjaer added to Brondby's misery.
By then it was the biggest away win since the Champions' League was created early this decade. Brondby's consolations were largely an irrelevance.
Now the Brondby manager, Ebbe Skovdahl, must lift his team for the daunting visit to Old Trafford on 4 November.
Even so, Alex Ferguson, the United manager, cautioned: "Brondby's best results have been away from home so we can't afford to be complacent. When we are complacent we don't perform properly but we owe it to ourselves to perform better at Old Trafford than we did here."
That comment reflected Ferguson's belief that, despite the margin of victory, "it was not a great performance. The finishing was very good but we were careless with our passing and made some mistakes. The conditions were difficult to play in. It was hard for Brondby to handle the suddenness of the goals.
"We have got the show on the road properly and the Bayern result opens up the league. It is a good opportunity for us - but I still believe if Barcelona beat us in Barcelona we will still go out."
Ferguson's mention of the difficult conditions were a reference to the driving rain and sodden pitch which caused problems for all players in keeping their feet, but especially defenders. Keane and Paul Scholes appeared to revel in the mud as they drove forward from midfield while Giggs, Cole and Yorke skipped lightly across the surface, exposing the sometimes ponderous Brondby defenders.
At the back the teenager, Brown, had such a composed European debut it looked as if he had played at this level for years. It was Brown who began the rout, running on to a pass from Keane and skipping down the wing before whipping over a cross which Mogens Krogh allowed to bounce off his chest. To his mortification Giggs was on hand to punish his error.
The goalkeeper, the understudy to Peter Schmeichel in the national team, had been at fault when Wales gained their memorable victory here 10 days earlier.
Giggs had been absent with injury from that Welsh victory but he seemed determined to show Danish fans what they had missed as he attacked at pace. After 23 minutes he also showed his growing aerial prowess as he rose above Brian Jensen to head United's second from a Jesper Blomqvist cross.
Cole had twice gone close from opportunities fashioned by Keane but he was more surefooted when, after 28 minutes, he moved onto Yorke's pass to add a third.
United were now sitting deep and counter-attacking with pace and purpose. Ten minutes before the break the Dane's night appeared to find a new low as Thomas Lindrup was booked for taking too long at a free kick. It seemed harsh since he was hardly likely to be time-wasting at 3-0 down and the punishment was compounded since it meant he will miss the Old Trafford return. But the decision provided Brondby with a rare silver lining as, from the re-taken kick, Kim Daugaard skidded a shot inside Schmeichel's near post.
That goal revived a crowd with fresh memories of Brondby's unlikely comeback victory over Bayern, but their hopes, like their skins, were doused 10 minutes after the break. Keane, seizing on the ball in midfield, drove through an opening, played a slick one-two and scored off the post.
The goal signalled another deluge to go with the inclement weather as Yorke and Solskjaer scored within the next seven minutes.
Yorke's goal was a well-taken header from Phil Neville's left-wing cross while Solskjaer, a substitute for Cole, scored with his first touch after good work by Keane and Yorke.
Ferguson also took the opportunity to rest Giggs and Yorke and gave a debut to the Scunthorpe-born Mark Wilson, the latest product of United's youth policy. There could not have been a better time for any teenager to make his bow and he settled in easily as United stroked the ball around as if at an exhibition match.
That was effectively what the game had become, even though Ebbe Sand tapped in Brondby's second in the dying minutes.
Manchester United (4-4-2): Schmeichel; Brown, G Neville, Stam, P Neville; Scholes, Giggs (Jordi, 60), Keane, Blomqvist; Yorke (Wilson, 65), Cole (Solskjaer, 60).
Brondby (4-4-2): Krogh; Colding, Rasmussen, Nielsen (M Jensen ,31), B Jensen (De Silva, 26); Bjur, Daugaard, Ravn, Lindrup; Bo Hansen (Bagger, 66), Sand.
Referee: V-M Pereira (Portugal).
Ince charged by FA, page 29