Chelsea were thumped 4-0 at Old Trafford due to sloppy defensive play and a high degree of naivety as their Premier League campaign got off to the worst possible start.
United soaked up the pressure created by Chelsea’s early bout of possession, but broke clear on numerous times, and showed the Londoners what finishing is all about.
Kurt Zouma gifted United a penalty with a terrible and needless challenge and then it all fell apart.
Frank Lampard will have to ensure the players’ confidence is not affected - next Sunday they host Leicester City at Stamford Bridge.
Chelsea had the almost perfect start, on four minutes, when Tammy Abraham thumped a venomous right-footer from just inside the United penalty area. He easily zipped the ball past De Gea but the ball ricocheted off his right post and was cleared.
However a Kurt Zouma loses ball allowed Antony Martial to try a first time shot. Thankfully Kepa Arrizabalaga snaffled the low ball.
Mason Mount, making his league debut for the Blues, tried his luck with a shot on 12 minutes, the ball took a slight deflection, but De Gea snatched the head-level effort.
But Chelsea defence proved it can be got at, and on 17 minutes Zouma, clumsily challenged Marcus Rashford inside the Chelsea area to give away a penalty.
Up stepped Rashford to smash a right-footer high and wide of Kepa’s outstretched right hand to put the home side 1-0 up.
De Gea hacked a Ross Barkley left-footer with his left foot six minutes on the clock in the opening half, as the visitors ploughed on for the equaliser. Seconds later Emerson then rattled the United crossbar.
There were no changes from either side at half-time.
The first effort of the restart fell to the Blues - a Pedro goal-bound left-footer, that took a slight deflection for a corner.
Frank Lampard brought on £58million Christian Pulsic for Barkley on 59 minutes. But despite holding the majority’s of the pressure, not going anywhere, but also failing failed to create a hole in the United defence, United broke quickly on 66 minutes and destroyed the Blues with two goals in two minutes.
First Martial prodded home from close range, them Rashford beat a high defensive line by Chelsea, outpaced Cesar Azpilicueta and rammed the ball home to give United a 3-0 lead.
Lampard opted to give N’Golo Kante a run out with 18 minutes left, Jorginho was replaced.
The fourth goal came on 82 minutes when on another break, United were cut apart, sub Daniel James saw his low drive, clip off Emerson and over Kepa’s distraught dive.
Mount saw a drive arrow towards the United goal, but De Gea was quick to react and that was as good as it got for the Londoners who languish second Fromm bottom of the Premier League.
Chelsea travel on Monday to Turkey to play Liverpool in the Super Cup on Wednesday.
Teams: Arrizabalaga, Azpilicueta, Christensen, Zouma, Emerson, Jorginho, Kovacic, Pedro, Barkley, Mount, Abraham
Subs: Caballero, Tomori, Alonso, Kante, Pulisic, Kenedy, Giroud
Man Utd: De Gea, Wan-Bissaka, Lindelof, Maguire, Shaw, McTominay, Pereira, Pogba, Lingard, Rashford, Martial
Subs: Mata, Young, James, Romero, Greenwood, Matic, Tuanzebe