Tuesday afternoon in Sant Joan Despí and there is bad news. A back injury looks like leaving Barcelona a man short when they travel to the Mestalla to face third-placed Valencia. But this time it's not Xavi who is missing; the midfielder is instead returning from injury. Nor is it Leo Messi or David Villa. It is Pep Guardiola, the coach. He is suffering with back pain and is not sure if he is going to make it.

When Guardiola's injury is conveyed to Valencia's manager, Unai Emery, he laughs. "Can't it be Messi with the bad back?" he says. Emery is joking. A chess and strategy fanatic, a man who can enthuse for hours on tactics and an admirer of Guardiola, Emery knows how important his opposite number is. But there is something in what he says – and not just because any coach would prefer to face Barça without Messi. As if to prove the point, Messi it was who scored the winner – and it was Messi again who set up Seydou Keita's tap-in in Barça's 1-0 win over Real Zaragoza last night.

In the end, Guardiola travelled. But he sat, awkwardly, rigid. Afterwards, he was admitted to hospital and a scan revealed a slipped disc. He could see the Camp Nou from his hospital bed but could not get there: it is not yet clear he will be there against Arsenal on Tuesday night or lead preparatory sessions. Many will ask the same question asked before the trip to Valencia. So what? It makes little difference, right? Wrong.

The impression has been fostered by Guardiola himself. Every time he is asked the secret of success he responds: "The players are very good." When Messi scored a wonderful goal at Real Zaragoza, he turned to a fan and said: "If it wasn't for Messi, I'd be coaching in the third division." Nothing to do with me, mate.

He is fooling no one. Guardiola's contribution is huge and has been recognised. He is lauded in Catalonia; "the legend is starting to be Guardiola himself," noted El País. A sports newspaper in Madrid tried to poke fun at his supposed perfection by sending his reporter to ask him a leading question. The question got exactly the desired response. "Maybe it's true," Guardiola replied, "maybe I do piss perfume."

But if Guardiola is given the credit for his work at Barcelona, there remain misconceptions that come with Barcelona's style; assumptions. Yes, they work hard now: "We had let ourselves go," Rafa Márquez said. But the way they play, well that's simple, natural. Autóctono, the product of 20 years' commitment to a footballing ideal, traceable to Johan Cruyff. Guardiola, captain under the Dutchman, said it: "This team will respect a philosophy," and one friend describes him as having "suckled from the teat of Cruyff". Xavi talks about the rondo – piggy in the middle – as the cornerstone of everything.

Which it is. But that makes it sound too simple, too unwavering. There has been much talk about how Arsenal will play Barcelona, but not very much about how Barcelona will play Arsenal. Well, the answer goes, like they always do.

Yes. But no. Under Frank Rijkaard, one insider claims, the exaggeration serving to make the point: "Barcelona found out who was in the team on the morning of the game." Guardiola could hardly be more different. Even as a player he was a coach, a thinker, a talker. "A talker?" says Fernando Hierro, the former Real Madrid captain, laughing. "He pretty much commentated the matches."

When he was offered the job in 2008, Guardiola asked his assistant, Tito Vilanova, if they were really ready. "Well," came the reply, "you certainly are." Charly Rexach, Cruyff's assistant, recalls that Guardiola was "the man we explained the tactical variations to. If we needed them, he implemented them." He had learnt too in Italy and in Mexico with Juanma Lillo, who coached in La Liga before he was 30. Guardiola had embarked upon a kind of pilgrimage – to meet Marcelo Bielsa, who has coached Argentina and until last month Chile, and the former's 1978 World Cup-winning coach, César Luis Menotti. The conversations lasted well into the night.

What some would describe as principle he believes is pragmatism. Guardiola designs his approach around the ball. Not because he is a puritan, although he is, but because like any other coach he wants control. Like any other coach, he is fearful and seeks to protect his team. It is just that his way of achieving control is different: defending well means attacking well. He will look at Arsenal and wonder how to protect himself from them, by trying to work out how best to do them damage.

"We play in the other team's half as much as possible because I get worried when the ball is in my half," he says. "We're a horrible team without the ball so I want us to get it back as soon as possible and I'd rather give away fouls and the ball in their half than ours." The stats bear that out: Dani Alves makes the fourth highest number of touches in the opposition half in La Liga. He is a full-back. Typically, only the two centre-backs and the goalkeeper spend more than 50% of the game in their own half.

Then there is possession: the top nine passers in La Liga are all Barcelona players. But that is not just an attacking option, it is a defensive one too. "There is no rule like in basketball that says you have to hand over possession or shoot after a certain amount of time, so 'attack' and 'defence' don't exist," Lillo says. Not in Barcelona's model. Barcelona attack to defend; when they lost to Arsenal, Guardiola was angry with Alves not for attacking too much but for attacking badly. That Barcelona lost because they were caught up the pitch is one reading; Guardiola's reading is that had they scored they would not have been caught on the break.

"Barcelona are the only team that defend with the ball; the only team that rests in possession," Lillo says. "They keep the ball so well, they move so collectively, that when you do get it back, you're tired, out of position and they're right on top of you." Lillo knows: his Almería side were defeated 8-0 by Barcelona.

Michael Laudrup, the Mallorca coach, said: "They move the ball so fast that by the time you get there, it's gone. You end up desperate, and shattered." As Rexach notes, Barcelona even waste time with possession. Most teams would go down to the corner; Barcelona would rather keep the ball between themselves.

In order to achieve that dominance, technical ability is fundamental, as is the pressure that is the coach's greatest obsession. But so is positioning. Barça's game is all about creating numerical superiority, opening up angles of passes. "We do a lot of positional work," Vilanova says. "That gives you options and prevents you from making unnecessary effort." Running, as Rexach famously put it, "is for cowards". "At Sevilla, you had to go looking for the ball," Keita says. "Here, it arrives at your feet."

Yet those fundamental lessons do not mean a lack of flexibility or invention. Nor does the faith in their identity mean ignoring the other team. Guardiola was accused by some of being tactically out-thought by José Mourinho last season or by Wenger at the Emirates. If so, it was not for lack of thought. Guardiola is every bit as obsessive a coach as, say, Rafa Benítez. "You wouldn't think so," Barça's reserve goalkeeper, Pinto, says, "but Guardiola controls every little detail." "Every decision is made according to the opposition," says one of Guardiola's collaborators. "Every one."

Messi's withdrawn role was initially employed – in 2009 – to confuse Real Madrid. Barcelona won that Clásico 6-2, Christoph Metzelder saying: "Centre-backs hate being dragged away from that position and we just didn't know whether to follow him out." As one of the staff puts it graphically: "With no No9 you leave the centre-backs to kick each other." Messi has now made that role permanent but not entirely inflexible. The reason is partly tactical, partly a response to the Argentinian's own desire. A different solution with Zlatan Ibrahimovic was aborted because of personal problems.

That means no Plan B – if by Plan B you mean a Big Man. But there are nuances and variations: plans C, D, and E. Besides, seeing tactical awareness only in terms of changing a game in course is a red herring; Guardiola would rather change the course of the game first; a successful coach ends up looking like a less interventionist coach. Against Athletic, he made his players receive on their own byline, four of them lined up around the area to receive from the keeper. "We knew they would pressure high and that risked us being dragged into long balls – which they would inevitably win," he said. Every move was 120 yards long. But if that's what has to be, so be it.

Against Valencia last week, there were three centre-backs and two wing‑backs. Within five minutes, there were also five long, uncharacteristic diagonals. The idea was to force Valencia to think twice about their high pressure. The plan did not entirely work – although Messi had countless chances – and this time Guardiola, suffering with sciatica, did make the change. On came Pedro. Messi got the goal; the assist came from Adriano, the man least expected to be included and the favourite to be removed.

There is a discernible Barcelona philosophy, a style. It is Guardiola's style, one so clear as to appear to suggest rigidity and insularity. The impression is not entirely true. When Guardiola travelled to South America, Menotti encountered a man who "reads, studies, listens and shows an enormous capacity for observation". And the observation is applied to opponents. Guardiola only knows how his team is going to play when he knows how the other team is going to play. Everyone has been asking how Arsenal will play at the Camp Nou this week. And Pep Guardiola is no exception.