To immediately understand the principal reason Betis humiliated Barcelona this weekend at the Camp Nou, you can go back to mid-July 2008 on a training pitch in St. Andrews, Scotland.

Yes, Ernesto Valverde was soundly beaten on tactics by Quique Setien.

Yes, if you compare Pau Lopez's superb first-half save from point-blank range, when Clement Lenglet thought he had scored, against Marc-Andre ter Stegen's limp-wristed effort that let Betis go 3-1 up, they can tell one tale of why the points went south instead of staying with La Liga's defending champions. And yes, we can also talk about William Carvalho giving a signature, man of the match performance -- Ivan Rakitic's red card comes into it -- and it's even fair to express constructive bafflement at Valverde's choice of substitutions, opting for personnel rather than tactical changes.

But if you're the type who wants to establish a single, dominant argument about why a team three points off the relegation zone ripped the champions apart and won at the Camp Nou for the first time in more than two decades, just refer to Pep Guardiola in the pale St. Andrews sunshine of midsummer 11 years ago.

He told a group of players, Gerard Pique and Leo Messi among them, "I can forgive you a misplaced pass. I can forgive you a missed penalty. We're all human. The one thing I'll never forgive you for is if you don't run."

Many things were germinated that summer, not just that dictum. But that speech was the beginning of not only a period of football that is arguably the best and most beautiful the world has ever seen, but also the football against which this group, unfortunately for them, will still be judged.

Wait: You think because Guardiola said that 11 years ago, it might not now carry the same weight or importance? That the world's best coach, if he'd been performing the autopsy on Barcelona's one-paced defeat to Betis, instead of winning the Manchester derby, wouldn't say the same?

You're wrong. So wrong.

Lionel Messi and Barcelona were exposed by Real Betis, who masterfully out-matched the title favourites in midfield. Urbanandsport/NurPhoto via Getty Images

Last May, having conquered England in record-breaking style, the Manchester City manager told Sky's Monday Night football programme something still more forceful and clear cut, something that should be hammering through the skulls of 90 percent of the Barcelona players who took the pitch Sunday against Betis.

Guardiola forcefully pointed out about his City squad: "I try to convince them to run like the most humble team in the world. If they don't do that, they don't play: They sit beside me on the bench. I can forgive everything, but if they don't run, they don't play. You have to run in football. That is one of our secrets: More than tactics, spirit, you try to run."

It's a vital point when analyzing Barcelona's current problems because irrespective of the power of the results on paper and their unarguable ability to go into overdrive most of the time when a "power play" is required, there are most certainly some massive problems. What happened at home to Betis this weekend can be traced, via football's family tree, to the 4-0 defeat in Paris a couple years ago, the 3-0 trouncing at Juventus a few weeks later and their excruciatingly bad aggregate defeat to Roma last season.

Still more worryingly for Valverde, it can be traced to the comeback win at Rayo Vallecano just a week before this 4-3 home defeat, admittedly Barca's first home defeat in la Liga in more than two years.

Even before making the crystal-clear point that Barcelona's Basque coach made a crucial error prematch, at half-time and then when he introduced further substitutions in "reading" how to combat Setien's tactics, it's imperative to accept that his players didn't, at virtually any stage, work hard enough in terms of shared defensive responsibilities.

Take the first Betis goal as one example. Then the second. At a stage when the visitors are in the process of cleverly, methodically building their way forward from their own penalty box after defending a corner, Junior Firpo is not a wing-back positionally; he's still a left-back, but he runs. Both Rakitic and Sergio Busquets allow him to run off them. They are either not bothered, despite Junior's similar run and goal last week against Celta, or they're complacent that "someone else will deal with it."

As the offensively gifted but defensively vulnerable 22-year-old -- Celta went from 2-0 down to 3-2 up last week in no small measure thanks to his lazy defending of the space in behind him -- who was making only his 25th La Liga start sprinted, neither Arthur nor Pique even thought about matching him.

Still, he was alone against a clutch of Blaugrana defenders, but because none of them helped out, it was effectively one vs. one: Junior against Sergi Roberto. The Catalan was slack (there's that word again). He didn't get tight and was unbalanced by Firpo's cut inside so that he'd have time to shoot past Ter Stegen.

Call it lack of energy, lackadaisical, complacency or "taking a rest" -- call it what you will, but it was precisely the threatening fallibility Barca showed for half an hour of the Clasico, from minute 45 until minute 75, when they went to 3-1 up and killed off a Madrid side that could have equalised or taken the lead, and for large parts of the eventual win against Rayo Vallecano.

More evidence, as stated, came for the second Betis goal. From my point of view, my diagnosis that Barcelona showed an urgent lack of urgency is damning.

Betis break from their own penalty area but are never closed down by an inattentive, somnambulant Barca. By the time Andres Guardado shunts a nice ball through space into the wide-open Carvalho, who's playing as if he thinks he's Zinedine Zidane, Barcelona are in numerical inferiority, with three of them facing four Betis players in the defensive third.

Lenglet, credit to him, races with 37-year-old Joaquin to make sure that it stays a numerical deficit of just one: It's then four vs. five. Less credit to Lenglet that he's 13 years younger and still made choices over the following seconds that allowed the beloved veteran to make it 2-0. No matter.

Who's missing from the picture? Malcom: Was he supposed to be part of a 4-4-2 when Betis had the ball? And, of course, Arthur, Busquets and Rakitic. Trotting, each of them. By the time Junior's cross isn't met, but Tello picks up the loose ball and centres for the unmarked Joaquin to score, the Betis "superiority" of numbers has gone to six Betis players vs. four Barca players. It's just wholly unacceptable from a champion team.

Marc-Andre ter Stegen was poor between the posts for Barcelona, something that didn't help their comeback attempts from 2-0 and 3-1 down. Eric Alonso/Action Plus via Getty Images

Anyone who says "Barcelona are defensively vulnerable" regarding their back four, based on that evidence, is missing the point. There's a line of Betis players there to potentially score, and the Barca midfielders who should be helping quell that threat arrive late on the scene: They're closing the stable door once the thoroughbred horse has already bolted.

Again, it's inarguable, I believe, that Valverde misread his team's ability to cope with Setien's 3-5-2, which could become a 2-7-1 formation if Joaquin dropped into midfield and any of the three central defenders stepped forward with the ball, while using his preferred 4-3-3 Barca formation. That is especially so with Malcom getting his Liga debut for a team whose tactical ideas he hasn't been given time to demonstrate that he understands.

It's a recipe for disaster. But it's still more damning that Valverde, last week, watched Rayo run and pass past his Barca players time after time, as if it were Olympians against statues. Also, did anyone on the Barcelona staff watch Betis 3-3 Celta, either live or on tape? Firpo's total unwillingness to be in the correct position when Celta probed the acres of space down their right wing resembled Marcelo's total disinterest in mundane concepts such as "defending" and "closing space."

Firpo is a fine athlete, a fine attacking player and a fine assist provider. He also has two goals in two games, each off a different foot. But he's ultra-vulnerable. Instead of Barcelona managing to make any capital from that, Junior rampaged past them, running them ragged, and with Tello on the other side, he must have wondered if one of the home players had been kidnapped or spirited away by aliens long before Rakitic was sent off.

It long appeared as if Betis had 12 or 13 players and Barcelona about nine. Valverde not changing Malcom for an experienced fourth midfielder and not asking Roberto and Alba to play higher, pinning the Betis wide players back with more bodies in the centre of the pitch to contest and even "own" possession, was simply bewildering.

But there's this. This was a Betis triumph born at the Camp Nou, not just executed there. I've long admired Setien for admitting, years ago, "I learned my concepts of football as a midfielder for Racing Santander chasing the ball and players of Johan Cruyff's dream team around the pitch and thinking to myself, 'When I'm a coach, my team won't play like this ... we'll play like them.'"

Kudos to him. Much as Guardiola would have disapproved of how little his former club ran in defeat Sunday, he'd have wholeheartedly approved of how Betis have inherited and successfully applied the Cruyff philosophy. It was, simply, beautiful.