In the end, it was a lapse in concentration as 679 minutes of Champions League football without conceding finished.
A most basic of errors for which, when the celebrations die down Massimiliano Allegri will no doubt pore over in video analysis. Joao Moutinho’s quick corner allowed the Portuguese to amble down the byline and drill a simple cross for Kylian Mbappe to score.
It was as un-Juventus a goal to concede as we could have witnessed; the established trio of Gianluigi Buffon, Giorgio Chiellini and Leonardo Bonucci supplemented over the course of competition by Mehdi Bentia, Daniele Rugani and, against Monaco, the timeless quality of Andrea Barzagli.
Before Mbappe’s intervention, Juve had successfully kept a pair of clean sheets against the highest-scoring teams in Portugual (Porto) and Spain (Barcelona) and for 159 minutes the same was true for France, and Europe’s, most potent.
It forms the bedrock for everything they do, as with such velocity on the flanks in Alex Sandro and Dani Alves and speed of thought in the minds and feet of Miralem Pjanic and Paulo Dybal, it allows them to be clinical in transition.
A Buffon save, Chiellini block or Bonucci interception and three passes later a chance has fallen to Gonzalo Higuain or Mario Mandzukic in the opposing area.
Assuming it is Real Madrid they face in Cardiff, it is a system that should suit against a side who tend to play on the front foot and don’t always leave the back door locked.
However, for all their defensive excellence in terms of numbers – fewest goals conceded, most clean sheets, joint second-fewest shots conceded – there were moments in the 180 minutes against Monaco, outside of Moutinho catching them cold, that should present a degree of concern before June 3.
Mbappe troubled them, not just with his physical attributes but his relentless desire to try and take defenders on the outside, even though all of his previous attempts had resulted in failure.
The saying when analysing two of the Italian greats of the past, Franco Baresi and Paolo Maldini, was they never had to make a tackle; their reading of the game ensuring anyone taking them on would invariably lose possession.
But Mbappe compelled Juve to tackle, and in doing so drew them out of position opening up space. More often than not there was a team-mate to cover, it’s part of the reason why they are so good but the gaps were there, on occasion.
Small details change games and while dominant, Juve were also a Buffon save and wondrous far-post Chiellini clearance away from
allowing Monaco into this tie.
Madrid don’t possess the same kind of out-and-out raw pace and power in an attacking player to take any of Juve’s defensive titans on man-to-man. Cristiano Ronaldo was once that but is now a penalty box predator who’s surging dribbling days are behind him, while Gareth Bale has always had the capacity but fitness is a concern.
They do, however, have their own Alves on the left-hand side in Marcelo, who’s been one of the best footballers on the planet of late, plus finishers who tend to take their chance in the biggest games.
Juve will have counter-measures ready, but Monaco have helped shine a flicker of light on a small chink in their awesome armour.