Barcelona were undone by a combination of Paris Saint-Germain's intense high pressing and a change in approach that has made the Blaugrana more vulnerable, writes Graham Hunter.
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When a team of such ability, pedigree and fierce competitive pride are beaten as comprehensively as Barcelona were overcome by Paris Saint-Germain on Tuesday, it would be foolhardy to suggest either that there was one single cause or that it was just a bad day at the office.
Barcelona were thumped 4-0 like this four years ago, at Bayern München in the semi-finals, with Xavi Hernández arguing not only that it was largely a product of Barça being in a worse physical state than their opponents, but also that there wasn't really that much between the sides after all.
Two seasons later, some of his points were substantiated when the Catalans eliminated Bayern in another UEFA Champions League semi-final en route to winning the treble. But this both feels and looks different.
The degree of superiority between Paris and the Spanish champions in this game was greater than in Munich in April 2013. Without taking any credit away from the victorious French title holders, nobody who has been watching Barcelona closely in recent months can actually be surprised that this has happened.
Luis Enrique introduced the concept that, in order to catch opponents out, it was a good idea to be less intricate, less possession-based in creating forward movements in matches.
That was because, as many of his players acknowledged, there were three extraordinary strikers up front who needed fewer openings and less of the work done for them before they could score.
So what, then, is the connection between what's gone on since that decision and this 4-0 round of 16 first-leg hammering in Paris?
Well, gradually over the months, Barcelona have enjoyed less possession, dominated games less, proved less able to exert control, and have been more and more susceptible to being pressed.
Often the brilliance in the Blaugrana squad can make Luis Enrique's side look not just healthy, but extremely dangerous. His reign has brought a flood of trophies.
But when a tactically superb, physically excellent, dynamic, confident and effervescent team like Paris truly go after this version of Barça, it's hard not to ask whether some of the clear-cut 'Cruyff' concepts, which Josep Guardiola held so dear, have become a little rusty.
Positional play is less important, the passing is more often at human, rather than super-human, speed, the pressing of opponents ebbs and flows. This has become a side that, when the front three are playing exceptionally, can beat anyone.
Yet they are also a team who, if under the cosh, suddenly have fewer 'systematic' remedies. And no Xavi any more.
To say it's impossible for Barcelona to turn round this knockout tie when their forward line is so exceptional would be to deny the history of this extraordinary competition. However, those who have followed the Blaugrana closely this term may not be holding their breath.