Earlier this season I was shown around the Sam Mames stadium in Bilbao. It's the oldest ground in Spain, nicknamed la Catedral, and is famed for its feisty atmosphere.
It echoes with history. It was in that arena that Bryan Robson scored for England after 27 seconds of a World Cup game against France. There is a statue of the legendary goalscorer Pichichi at which opposing captains lay flowers the first time that their club visits and there is the dugout from which Fred Pentland, Ferdinand Daucik and Javier Clemente masterminded league triumphs.
We walked through the dressing room, pausing in front of a plain white door in the corner. "That's the coach's room," said our guide in an awed whisper. "Before I'd have been able to show you there, but now... well, Bielsa."
The night before I'd seen Athletic eviscerate Paris St-Germain. It was only 2-0 but for the hour until Momo Sissoko was sent off and Athletic declared, it had pummelled a far more expensively assembled side, landing blow after blow. That was Athletic's first home win under Marcelo Bielsa and it was greeted with a sense of relief. Three days later he oversaw his first league win, two magnificent Fernando Llorente goals giving Athletic a 2-1 victory at Real Sociedad in the Basque derby. The Bielsa revolution was up and running; it has now taken Athletic to Wednesday's Europa league final.
There had been scepticism. Joaquin Caparros, his predecessor, had instilled a long-ball style different from Bielsa's philosophy and had taken the club to sixth in the league and to the final of the Copa del Rey.
While there was some frustration at the directness of his approach, there was also a sense that it was in keeping with Athletic's traditions: since the golden age under Pentland in the 1920s it has always prided itself on being the most "English" of Spanish clubs (hence the use of 'Athletic', the name the club was given by the British shipyard workers who founded it, rather than Atletico).
Scott Oliver even argues in Issue Four of The Blizzard that the term "tiki-taka" for Barca's short-passing approach was initially coined by Clemente, who led Athletic to two league titles in the early eighties, as a term of derision. Yet Pep Guardiola, elevated to the status of high priest of tiki-taka in his tenure as Barcelona manager, readily cites Bielsa as an inspiration. When they first met, their conversation went on for 11 hours.
Bielsa has become a cult. Gerardo Martino, Jorge Sampaoli, Mauricio Pochettino and Eduardo Berizzo all happily speak of themselves as his disciples - as, intriguingly, does Diego Simeone, the coach of Athletic's opponents Atletico Madrid on Wednesday, although the stylistic influence is far less obvious. There is even a sense that so remote can he seem at times form the hurly-burly of a game, that Bielsa is a greater thinker than he is coach.
He is eccentric and brilliant, supposedly always takes 13 steps from the bench to the edge of the technical area, wears a string on his glasses and spends most games either squatting or raging - and occasionally both - in an ill-fitting tracksuit. He refuses to address the media in anything other than general press-conferences, affording a journalist for the smallest local paper the same status as the anchor for the biggest TV station.
In those press-conferences, though, he talks and talks, often for several hours. His answers, usually delivered in a low voice with eyes downcast, are invariably rambling, sprinkled with a jargon of his own creation, barely comprehensible and laced with genius. The satirical Catalan TV show Crackovia recently showed him responding to a cheery greeting from the Barcelona coach Pep Guardiola with a five-minute lecture in which he deconstructed the words "good morning".
Is he as mad as he looks? "Oh, no," said the Athletic forward Iker Muniain. "He's madder."
Bielsa was born to a family of lawyers and politicians in Rosario, Argentina; his brother, Rafael, is a deputy for Capita federal while his sister Maria Eugenia used to be vice-governor of their home province of Santa Fe.
From an early age he was taught to revere books, study and analysis. He took that approach and applied it to football. Never more than an average defender, he took up coaching at the age of 27. His first job was with the University of Buenos Aires; he watched 3,000 players before selecting his 20-man squad.
He obsessively watches matches, analysing and annotating, creating what is effectively a taxonomy of football. That's what he does in that locked room off the dressing-room.
Before he had even arrived in Bilbao he had watched all 55 games Athletic played last season, 42 of them twice. "There are 36 different forms of communicating through a pass," Bielsa insists. He once drew on his shoes to explain which part of the foot he wanted players to use and carried on wearing the same pair for days, pen marks still visible. Video sessions can last hours, players kept alert by the dread that the light of Bielsa's green laser will land on them and they will be asked to explain something on screen.
In training, moves are pre-practised to be put into effect in game situations. One drill involves a grid of nine squares in which 10 players pass a ball, constantly moving to avoid there ever being more than one man in any square for more than a second or two. Possession and movement are the cornerstones of Bielsa's philosophy with the ball; pressing when out of it. He hates sideways passing for the sake of it, though, his press-conference being peppered with calls for "vertical penetration".
He has adapted to Athletic, his desire for verticality finding an echo in the club's traditional directness. After some disappointing early experiments, the 3-3-1-3 he preferred from his days at Newell's Old Boys, with whom he reached a Copa Libertadores final only to lose on penalties to Sao Paulo in the early nineties, to his time as Chile coach, has disappeared to be replaced with a 4-2-3-1.
The obsession with possession endures to the extent that he often plays a holding midfielder (Javi Martinez) at centre-back, but he is prepared to use the aerial strength of Llorente and use crosses and long balls on occasion. The result is a side that plays some of the most thrilling and dynamic football in Europe. Athletic may have run out of steam in the league, but its two wins over Manchester United will not quickly be forgotten, either in Bilbao or Manchester.
The task now is to finish the job, to win Athletic its first European trophy and to win Bielsa his first continental triumph, and prove that he is not only a great theorist but also an excellent coach.