Goals from Pedro, Gary Cahill, Eden Hazard and N'Golo Kante resulted in Jose Mourinho's heaviest defeat since first arriving in the Premier League 12 years ago.
The Red Devils were abysmal at Stamford Bridge, conceding four sloppy goals after some awful defending from the likes of Chris Smalling, Daley Blind and Paul Pogba.
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Chelsea's opener was particularly embarrassing for United. Marcos Alonso's ball over the top evaded both Smalling and Blind to find Pedro, who rounded David de Gea and finished with aplomb.
The Spanish winger's strike came within just 30 seconds and a hilarious video has now gone viral of the goal playing out to the sound of Countdown (see below).
"CHELSEA DOES COUNTDOWN"
Perfect timing. What makes the video even better is how Countdown's 30-second timer finishes just as the camera pans on a glum-looking Mourinho on the sidelines.
The Portuguese was understandably disappointed with how his players dealt with each of Chelsea's four goals, attributing them all to "incredible defensive mistakes".
He said to Sky Sports in a post-match interview: "In the first minute, you analyse the goal and you can see what happened - we made incredible defensive mistakes, individual defensive mistakes and you pay for that.
"After a few seconds you are losing 1-0, then you go close to 1-1, then another defensive mistake and it's 2-0.
"Then you are close to 2-1 and they counter for 3-0, then you are close to 3-1 and they score to make it 4-0.
"You come with a strategy and you cannot concede a goal the way we did in the first minute, that changed the game."
It's now back to the drawing board for Mourinho ahead of Wednesday's EFL Cup clash with Manchester City and next weekend's game against Burnley, both at home.
Defeat to Sean Dyche's man on Saturday would spell disaster for United, who have secured just five points in their last six Premier League games.
They currently sit seventh and six points adrift of joint league-leaders City, Arsenal and Liverpool.
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