It is impossible to know whether this Messi has already recorded one of those defining moments. Perhaps it will, in time, prove to have been that chip against Real Betis a few weeks ago, the ball floating for an age before arcing over the line.
It is one of the hallmarks of Messi’s greatness that so much of it is so serene, so peaceful. There is rarely any anger to his play: more the ruthless grace of Roger Federer than the explosive strength of Rafael Nadal. He caresses his passes and strokes his shots, control always prized above power.
The other is that he does not abuse his ability: He rarely indulges himself with impossible shots from distance, seeking headlines and limelight and acclaim. His software is now sufficiently sophisticated that he can seemingly calculate the odds of any given decision: He shoots only when that is the correct decision.
His first goal here on Tuesday was a case in point. He nutmegged Fred on the edge of United’s box — not because it drew the biggest gasp from the crowd or because it showcased his genius, but because it was the simplest route to the spot, just outside the penalty area, from which he could whip the ball around David De Gea. (His second goal does not warrant such examination — a soft shot that De Gea, uncharacteristically, fumbled: Even the greats are allowed to get lucky.)
Maybe that goal will prove to be the high watermark of this version of Messi; maybe it is still to come. He produces brilliance with such astounding frequency that only with hindsight — and a considerable amount of it, too — is certainty possible.
An example: if Messi had been able to finish off a run, late in the first half, that took him past three United midfielders, the referee Felix Brych, and left Phil Jones twisted and turned and tortured, his first goal would have seemed fairly ordinary, by his standards; so, too, if he had managed to convert an impromptu scissor kick in the second.
The question, now, is how far that brilliance can carry Barcelona. An eighth Spanish title in 11 years is nearly secure already a run of domestic success unparalleled in the club’s history but a first Champions League semifinal appearance since 2015 is, arguably, of greater significance.