It would go on to become the Mohamed Salah show yet again - the Egyptian bagging a stunning a hat-trick, all three goals with his left foot that has proved so deadly in recent weeks.
Things somehow got even worse for Ole Gunnar Solskjaer's side after half-time substitute Paul Pogba was sent off just 15 minutes after being introduced for a high challenge on Keita.
Both the score-line and the manner of the result will undoubtedly lead to fresh scrutiny surrounding Solskjaer's future as manager, despite his recent three-year contract extension.
Here are four things you may have missed as Liverpool ran riot at Old Trafford.
Cristiano Ronaldo kicks out
Rarely has Cristiano Ronaldo every endured such a brutal drubbing – the Portuguese icon trudged down the tunnel at half-time as his side found themselves 4-0 down with a look of disbelief.
One to often get his own way, Ronaldo was unable to contain his frustration as the first half drew to a close.
Aiming to keep the ball in play against a Curtis Jones who was intending to simply marshal the ball over the line for a goal-kick, Ronaldo wrapped his right-foot around the legs of Jones to foul the Reds midfielder.
Taylor blew his whistle – sparking a furious response from the forward, who booted the ball into the groin of Jones, sparking a heated exchange between players from both sides who knew full well what Ronaldo had done.
The United number seven can consider himself fortunate to escape with just a yellow card, though perhaps have wanted to get off the pitch as soon as possible.
Harry Maguire’s glare at Luke Shaw
In the immediate aftermath of Liverpool ’s second, Luke Shaw played Harry Maguire a particularly poor pass that left the United skipper with little option but to desperately prevent a corner.
Maguire’s subsequent clearance resulted in a Liverpool throw-in on Shaw’s left-hand side – the nervous tension in the ground at its peak during this exchange.
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Whether Shaw’s decision to not look back in the direction of his captain stemmed from wanting to be ready for any impending Liverpool attack or just fear of the telling off he would get from Maguire is unclear – but what the left-back didn’t see, millions at home did.
The camera's picked up Maguire giving his teammate an intense look that barely hid his views on Shaw’s pass.
Salah’s punches of frustration
Mohamed Salah has had very little to be angry about in recent weeks – the Egyptian receiving praise from just about every section of world football, many dubbing the Liverpool forward the best player on the planet.
Salah kept up his fine vein of form when the Reds needed him most as he became the first Liverpool star to bag a goal on three successive visits to Old Trafford – the tenth game in a row he has found the net this season.
However, he did not have it all his own way at the Theatre of Dreams - latching on to an overhit and frankly quite bizarre back-pass from Marcus Rashford, Salah looked set to score even earlier than he did, only to see his effort saved by the chest of De Gea.
Though a difficult chance, no one was more annoyed with the miss than Salah himself – fiercely punching the turf in fury as play continued up the other end of the pitch.
If only Liverpool’s number eleven had known he would find the net a brace before the half was out.
United fans leave in their droves
Having just witnessed their side go in at the break 4-0 down for the first time in Premier League history, it is somewhat unsurprising that many of the United faithful saw fit to leave Old Trafford early.
Television coverage displayed what appeared to be hundreds of United fans head for the exit before the whistle had blown for the second-half.
The last time United found themselves 4-0 down at half-time at home came 70 years ago at Bolton Wanderers in a contest that eventually ended up 5-1.
While fans who leave early are often left kicking themselves when their team sparks an incredible comeback, it was not to be tonight – Salah securing his hat-trick to make it five within minutes of the restart.Read More Read More