Pep Guardiola looked his inquisitor in the eye and replied without hesitation: "I would never, ever sell him and I wouldn't swap him for anyone." Nothing unusual there, you might think, but Guardiola was not talking about Leo Messi, Andrés Iniesta or Xavi Hernández. He wasn't talking about Dani Alves or Sergio Busquets; he wasn't even talking about Cesc Fàbregas. He was talking about Javier Mascherano. And his words were not empty ones. Guardiola meant it. It is time to revise those easy assumptions.

The search for a weakness in the Barcelona side that Chelsea face in the first leg of their Champions League semi-final this week often settles on the midfielder playing at the back, on the player who does not appear to fit the Barcelona philosophy. There is just one problem with that argument: it is not true. Not any more. Mascherano has had to relearn his trade; in fact, he has had to learn a different trade and he has had to be patient, waiting for his opportunity, accepting his fate. But he has started 24 times in the league and in seven of nine Champions League games, and become so important to Barcelona that the assumption that he will return to the bench the day everyone is fit no longer looks so secure.

There were lots of people who could not really see how Mascherano would fit into the Barcelona team when he signed from Liverpool in the summer of 2010. Mascherano was one of them. He had signed only after the Catalan club's pursuit of Fàbregas failed and his debut ended with a shock 2-0 home defeat by Hercules, in which Mascherano – slow on and off the ball – gave away the free-kick that led to one of the goals and was fortunate to escape a red card. It also ended at half-time, when Guardiola took him off.

Few doubted he was a good player; many doubted he was the right player. Others wondered how long he would accept secondary status; this, after all, was a €22m signing and captain of Argentina. El Jefecito, the little chief. He was also 26 years old, not a kid with time on his hands. The answer, as it turned out, was: as long as it takes. "I'd be an idiot if I thought I'll play ahead of Xavi and Iniesta," Mascherano said. As for Busquets, the man occupying his position, Mascherano called him "the perfect player".

"It is a blessing to be here. I watch Busquets and learn," he said. "I'd love to be able to play like [Xavi, Iniesta and Busquets], but I can't."

The Argentinian might not have won a place immediately, but his attitude quickly won people over. He has filled in whenever he was called upon and kept quiet whenever he was not. Instead of wailing, he watched. When he speaks he does so with clarity and conviction, a quiet authority, and with rare understanding and analysis. Yet there is also a kind of wide-eyed eagerness and he is never off message. He speaks with a convert's zeal, describing team-mates with a kind of hushed awe. "I have never seen anything like Barcelona; it is not that they win, it is the way the win," he said. "I now know that there's another way of thinking about football.

"I have come here to learn and enjoy," he insisted. Barcelona can be likened to a sect in its single-minded, exclusive commitment to an ideal: Mascherano has successfully been re-programmed. "Pep always surprises you with some detail," he says. The broad brushstrokes, the fundamentals of the central midfielder's role, have been altered too. Always taught to follow his passes, Guardiola preaches the opposite: moving away or simply stopping still to open up new lines of passing. Rather than resisting, Mascherano received the advice like a revelation. "I run less, but I am closer to the play," he says. "Positioning is vital here."

If his positioning has changed; so has his position. However much he developed, Busquets ensured that midfield opportunities were limited. With injuries, opportunities at the back have opened up. Mascherano has played as one of two centre-backs and was flawless in that role during last year's Champions League final. But in the absence of Eric Abidal and with Alves alternating between right-back and right winger, it is his role in a three that has really brought him to the fore. "It's not easy adapting and it's been strange to leave the midfield, my natural role, but I feel comfortable with Piqué and Puyol," he says.

The feeling is mutual. Quick across the turf, neat and simple in possession, and handed the role of leaving the back three to seek to win the ball, striding out if successful, returning if not, he has been a revelation. Against Milan and Athletic Bilbao, Barcelona's last two home games, Mascherano committed only one foul, yet he recovered possession 27 times, averaging more than anyone in the Barcelona team. Over those two matches he only lost the ball once. Speed helps; awareness too. For all the intensity and toughness, intelligence has been the key to his success.

Asked which of his team-mates would make the best manager, Alves insists: "Mascherano. He has a spectacular view of football. Whenever I talk to him, he tells me something new; he notices things you don't. He really understands the game."

"I never expected him to give us as much as he has: his value is unimaginable," Guardiola admitted. "He has been spectacular as a player and a person. Javier Mascherano is the best signing this club has made in four years."