This was the first time Manchester United had beaten Liverpool under Ole Gunnar Solskjaer, and while the immediate result was to knock them out of the FA Cup, there may be greater ramifications for the rest of the campaign.
Psychologically, this could be important, with Liverpool playing well at times only to have their obvious vulnerabilities exposed, while United's talisman, Bruno Fernandes, came off the bench to score the stunning free-kick which secured a fifth-round tie at home to West Ham United.
Liverpool are out and while this competition may not be a priority in terms of their greater ambitions, Jurgen Klopp fielded a strong side, as did Solskjaer, to acknowledge the potentially wider resonance of the result as well as the magnitude of the opponent.
Instead it is just one win in seven for Liverpool - against Aston Villa's kids in the previous round - with again serious concern about the centre of their defence.
With Klopp searching for solutions, the brutal truth is 19-year-old Rhys Williams is not the answer, not right now anyway, while Fabinho was also at fault. Can Liverpool really afford not to buy a centre-half before the transfer window closes?
Undeniably it is coming together for United with Solskjaer's tactics, attacking the space left by Liverpool's full-backs proving effective, Paul Pogba again outstanding and Fernandes just seems to lift his team-mates when he is on the pitch.
Belief is coursing through them, while the Portuguese has that wonderful, charismatic presence that few players possess.
There is no secret with Solskjaer revealing the playmaker had stayed out on the training ground to practise free-kicks when he was told he was not starting this tie.
And so it came to pass. Just as Liverpool looked the more likely to win, after Mohamed Salah scored his second goal, Edinson Cavani cleverly rolled Fabinho on the edge of the penalty area to win a free-kick which the Brazilian angrily protested.
Cavani inquired about taking it but Fernandes made it clear what was happening and he struck the ball fiercely around the wall - with, crucially and inexplicably, Thiago Alcantara ducking - to beat Alisson.
This was far more open and attacking than last weekend's Premier League stalemate at Anfield and it was Liverpool who took the lead.
After not scoring in their last four league games, with their only goals coming against Villa's third string in the previous round, it was sumptuously constructed and taken.
Roberto Firmino collected possession, there was no pressure on him and he took advantage with an incisive pass between Victor Lindelof and Luke Shaw for Salah to run on to and calmly lift the ball over the onrushing Dean Henderson and into the net. Lindelof was at fault, having lost Salah, as was Shaw who was slow to react.
In a sign of how open the tie would be, United quickly hit back with Pogba determined to win the ball on the edge of his own area, taking it off Firmino who fouled him with referee Craig Pawson playing on as Donny van de Beek quickly found Marcus Rashford.
Salah sprinted back to close Rashford down but he smartly switched play with a precise cross-field pass over James Milner's head for Mason Greenwood to chest down. As the 19-year-old steadied himself Liverpool defenders tried to get back but Greenwood drove the ball unerringly across Alisson to equalise.
It was as good as Liverpool's goal and was a definite ploy from United to try and get Greenwood in behind Andrew Robertson and Rashford occupying the space left by Trent Alexander-Arnold.
It worked with Greenwood's pace having taken him away from Thiago, forcing Alisson to save, and Cavani seizing on the first of several errors by Williams for Greenwood to shoot wide.
There were more chances - with United dominant at corners - before Rashford skipped past Williams, who hesitated and then dived in, for Greenwood to find Pogba who shot wide.
Williams appeared nervous and United profited when Cavani fed the ball to Greenwood who struck a first-time pass towards Rashford. The Liverpool centre-half should have cut it out but fluffed his chance with Rashford running on before side-footing beyond Alisson.
At that point in the second half there appeared to be only one winner but the goal flicked a switch with Liverpool who will be encouraged by their response.
Milner went close when he reached Firmino's header on but could only half-volley over at full stretch.
However he was heavily involved as Liverpool drew level. The midfielder intercepted a lazy pass by Cavani to surge forward and then intelligently dummy a low cross from Firmino, suddenly prominent and knitting the play the way he used to, to allow Salah to sweep the ball home.
Both managers tried to seize the initiative. On came Sadio Mane and then Fernandes and Fred and it was Liverpool who threatened again - Firmino threading the ball through to Salah, whose low shot was parried by Henderson, with Harry Maguire fortunate when his poor clearance struck Lindelof on the back and bounced away.
Just as it looked like Liverpool were going to overwhelm United it was the home side who profited through Fernandes' free-kick. Cavani used his nous to create the chance and then went close himself to scoring when he met Fernandes' precise cross, only to thump a header against a post.
Liverpool could not take advantage and it was United who went through.
Oddly, it means that only once in Klopp's six seasons in charge has he reached the fifth round of this competition. But, to be honest, that is the least of his concerns right now.
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