Leaving home in the Argentinian town of Murphy at 14 helped develop his temperament but it was rapid promotion amid a crisis at Espanyol that showed what he could do

It was one o'clock in the morning when they knocked on the door. It was cold outside in Murphy, Santa Fe province, and Mauricio Pochettino was fast asleep. His parents, though, ushered the two men into the house and they crept quietly up to the 14-year-old's bedroom, pulling back the covers and waking him with a start. "He looks like a footballer," Marcelo Bielsa said, peering down at his legs. Alongside him, the former Atlético Madrid player José Griffa smiled. They worked as scouts for Newell's Old Boys and they had come to sign him. It was 1987.

Bielsa later admitted to Pochettino's son that, far from looking like a footballer, his dad actually looked a little chubby. But 12 years after that first meeting player and coach worked together in the Argentina team and before that they had won the clausura for Newell's. Pochettino was a central defender and his talent had been obvious as a kid but it was his temperament that really set him apart: the dedication, the will to win, the fearlessness. The sheer ambition. It still does.

"People see players and say: 'He's 20-years old and he's got a great car,'" Pochettino said in 2009. "Perfect. But what has he left behind? I left home at 14 and lived in a pensión where I shivered at night and couldn't sleep. At that age, lots of kids still live at home with their mums, who tuck them in at night."

Pochettino played for Espanyol, twice, Paris Saint-Germain and Bordeaux, as well as representing Argentina. He had been in France for less than a year when he was made captain at PSG. "And that," says one of his closest collaborators, "is a detail that says much about him." At Espanyol, he made more appearances than any foreign player ever, becoming captain there, too, and twice won the Copa del Rey, in 2000 and 2006. No one carried such weight in the dressing room, one former team-mate describing him in three words as "confidence, security, ambition". And few connected better with the fans. He became an idol, a man who says with a smile: "My kids sleep in Espanyol pyjamas."

Paco Flores was the coach when Espanyol won the 2000 Copa. "Pochettino had great charisma in the dressing room," he says. "He never, ever accepted defeat and there was a huge amount of respect for him, almost like the hierarchy when you're doing military service. He really motivated his team-mates. He was a leader, a strong personality. There was also a powerful communion between players and fans and he saw the importance of that. He used that very well."

When it came to taking over as coach, that helped. Pochettino's arrival at Southampton was met with scepticism and rejection: the sacking of Nigel Adkins struck supporters as unfair and it was natural that some might see a usurper in the Argentinian. At Espanyol, by contrast, he was welcomed as a potential saviour. But it was far from easy. The club were in crisis, "on the very edge", in the words of one member of the technical secretariat. "Risky?" Pochettino said. "Everything in life is a risk. I can take that on."

It was week 19 and Espanyol had just sacked their second manager of the season: Bartolomé Márquez had lasted 13 games, Mané just six. Pochettino was a football fanatic; as a player he and Iván de la Peña would meet after training sessions and watch whatever game was on, no matter what level or what league. He had got his coaching badge only two months earlier and as part of his prácticas he was working as assistant manager of Espanyol's women's team.

There had been suggestions that Pochettino might take over alongside Xabier Azkargorta, who is now Bolivia's manager, and rumours even circulated that, in the midst of Espanyol's institutional crisis, he would fancy putting himself forward as a presidential candidate. Neither happened. He told the president, Daniel Sánchez Llibre, that his ambition was to be first-team coach. A process that was already under way, at least vaguely, was accelerated. Here he was, first-team manager with no real experience. But if there were doubts there was no rejection; instead they embraced him.

It was, says Flores, a natural progression. "We used to discuss tactics a lot; there would be debates and you always got something out of it. He was meticulous and above all very ambitious. On the pitch, he was the coach's arm." Besides, circumstance dictated. "The idea had been for Pochettino to take over the second team the following season," says Ramón Planes, who was made sporting director at the same time that Pochettino was made coach. "But the situation was so dramatic that he was made first-team coach. He was the only one that could turn it round. He knew the players, many of whom had been team-mates of his. He could have ascendancy over the dressing room. And he said he was ready."

That process was not without its difficulties: Pochettino had a different role to play and for some of the older players, the transition from team-mate to coach proved uneven. Economic and footballing reality played their part too: Espanyol's budget virtually halved in two and a half years. In that time, under Pochettino almost 20 players came up from the youth team to play for the first team. For some that was "brave", for others it was a problem. Younger players offered him more aggression, intensity and hunger, one veteran says. But, he muses, the other good thing about young players might just have been that they're more malleable.

"As a team-mate it was one role; as a manager, another," says Moisés Hurtado, who played with Pochettino and then under him. "As a player he had weight in the dressing room but as a manager there were later some attitudes that I didn't share. He wanted to control everything. The first season was fine: he'd been a player and he understood, he connected with us well. But then things changed. He seemed to see conspiracy where there was none and some good people had to leave out the back door, and not just players. He wanted everyone to dance to his tune, people entirely committed to him. The atmosphere ended up not being so good. In purely sporting terms, though, there was no problem: he got great results and we played well."

Slowly, the veterans departed. Pochettino had warned that there would be no favouritism. "It doesn't matter who the person is," he said when he took over. "As a player I was demanding; as a manager I will be too."

He was true to his word. He would turn up hours before everyone and leave hours after. "That makes it easier for players to accept the demands," one member of the staff says. "They think: 'Hell, if he's doing it …'"

Sessions were intense and innovative. Players' shirts were fitted with GPS devices and connected to a programme that monitored their every move. A camera filming the game on a wide angle linked directly to an iPad on the manager's bench. He would not just tell players what had gone wrong, he would show them.

Sessions were carried out with movements co-ordinated exactly. Pochettino recalled the time he scored a header as a player but Bielsa tore into him for being in the wrong place at the time. Pochettino, too, liked to control every detail but Planes insists that he did not reach the point of obsession. "Mauricio gives players the freedom to express themselves. You might see Jay Rodriguez on the other wing and that's no problem. He sets clear parameters but, unlike some coaches, gives them freedom. He gets on very well with the players; he takes the pressure off them and makes them feel free. He's not a policeman."

"The most important thing is to defend well and build a structure from there," Flores says, "but despite being a defender Pochettino wants his teams to attack. In a way, I think that's normal: you try to perfect the things that as a player you lacked. As a defender you know what kind of attack you hated facing and you try to apply that."

Hurtado says: "He wanted the centre-backs to open out and the midfielder to drop between them to receive the ball. He wanted us to play."

Pep Guardiola once said that he identified with Espanyol's football.

Planes adds: "Mauricio always wanted his teams to treat the ball well, to be very proactive, not just to wait for the opposition. He wanted his teams to be dynamic, brave and attacking – and that's still his approach. He wanted the pace to be high and for the team to really press." He insists, though, that Pochettino is not a disciple of Bielsa. "I spoke to him about football a lot and I don't think he had one single model."

Above all, the demands were huge. "Tell me how you train and I'll tell you how you play is a Spanish phrase," Flores says. "Well, Pochettino the player and Pochettino the manager took that to heart." His players talk about "hierarchy", "intensity", "effort", "seriousness" and "discipline"; exactly the qualities that define the manager and always have – right back to that bedroom in Murphy.

Daniel Pablo Osvaldo played with Pochettino at Espanyol and joined him again at Southampton. "He makes you suffer like a dog," the forward once joked. "And at the time you hate him for it but by the Sunday you're grateful because it works."