At the time, nobody was aware quite how defining a moment it would be.
But, like all the greatest footballers, Lionel Messi knew the potential significance. He knew it all too well.
Having raced away during injury time at Camp Nou last week, Messi laid it on a plate for substitute Ousmane Dembele to score a fourth and surely put Liverpool to bed.
Thankfully for the Reds, Dembele finished horribly, the chance gone. A 3-0 defeat was bad enough, but 4-0 would have been insurmountable.
No wonder Messi beat the turf in frustration at the failure of his team-mate.
“It was very clear, it would have been better to finish with four than with three,” said the Argentina international after the game last Wednesday.
“We know the game isn't over, we know we're going to a very difficult stadium with a lot of history, where the fans are really behind their team."
Messi knew. He knew it all too well.
So when Divock Origi struck the fourth goal to send Anfield into delirium, the Barcelona man could be spotted with his hands on his hips, wondering why on earth his team-mates had wilted in such remarkable fashion.
Barcelona were well beaten by then, a spent force unable to muster much of a response to going behind on aggregate.
Messi, to his credit, kept on plugging away. But with Liverpool, primed by Fabinho, locking him down, the Argentine was, to a large extent, neutered.
Luis Suarez knew precisely what to expect - describing the fans as Liverpool's "12th man" in the build-up - but even he was powerless to repel the power of Anfield. Philippe Coutinho, for whatever reason, didn't even bother an attempt.
Messi was only 19 on his previous appearance at Anfield, on the winning Barcelona side on the night but beaten on aggregate in the Champions League first knockout round in 2007.
On Tuesday, though, he was spotted as the teams lined up for the Champions League anthem, taking a glance at the swaying, baying Kop.
But it wasn't just the crowd that Messi feared would make an impact.
After the first leg, he admitted to the Spanish press he was "exhausted" by coming up against a relentless Liverpool side, whose superior fitness and energy levels told during the final quarter at Anfield.
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Messi knew Liverpool would not give up.
Just like, if it were needed, Manchester City and their manager Pep Guardiola - a former boss of Barcelona, don't forget - know the Reds won't surrender their Premier League title challenge on Sunday.
With one miracle down, Liverpool need another, trailing the champions by one point with that single game remaining. But the Reds are in no mood to make it easy for City.
This remains a championship that will be won rather than lost.
George Sephton, the voice of Anfield, played out the evening with John Lennon's Imagine.
Liverpool can still be dreamers, on two fronts. Messi was right - they just don't know when they are beaten.
