"The third year," the great Hungarian coach Bela Guttmann always said, "is fatal." If a manager stays at a club more than that, he said, his players tend to become bored and/or complacent and opponents start to work out counter-strategies. There are occasional exceptions, especially in weaker leagues, but at the highest level it seems to hold true that great teams last a maximum of three years – which is why Barcelona's draw against Espanyol on Saturday may be more significant than just two dropped points. This, after all, is Pep Guardiola's fourth season as a manager at the Camp Nou.

Guttmann's solution was to keep moving, collecting bigger and bigger signing-on bonuses, never hanging around long enough for entropy to set in. The other solution is for the manager to stay put and for the players to change, which is the strategy Sir Alex Ferguson has employed in his unprecedentedly long spell at Manchester United: he has, essentially, had a core of loyalists who seemed immune to complacency (perhaps because having grown up in United's youth sides they knew no other way – Gary Neville, somebody at Sky mentioned recently, is apparently determined to be the best pundit on television presumably because he is programmed to work with ferocious discipline to be the best at whatever he does), and has supplemented them with a rotating cast of imports. Any player who has showed the slightest sign of wavering from the one true path has been ruthlessly dealt with. The most notorious cull, of course, came after United had finished second in the league and lost in the FA Cup final in 1995, leading to the departures of Mark Hughes, Andrei Kanchelskis and Paul Ince.

(The Real Madrid side that won five European titles between 1956 and 1960 may seem an obvious counter-example, but they were in a state of permanent evolution. Only four players who played in the 1956 final also played in the 1960 final, and they had six changes of coach in that period – although only four different coaches).

The three-year rule seems particularly to apply to sides who play a hard-pressing game. Viktor Maslov, the father of pressing, led Dynamo Kyiv to three successive Soviet titles between 1966 and 1968, but was removed from his managerial position in 1970. Ajax won a hat-trick of European Cups between 1971 and 1973 (a period in which Rinus Michels departed to be replaced as coach by Stefan Kovacs), before Johan Cruyff's acrimonious departure after other players voted for Piet Keizer to take over as captain. Arrigo Sacchi's Milan won a scudetto and two European Cups before the sheer effort, mental and physical, of maintaining the hard-pressing approach overwhelmed them.

Decay can have numerous causes. A shape-based game saps players because it requires constant thought, and because training to get the shape right is boring and repetitive. Pressing is physically exhausting, demanding perpetual running. Players living and working in close proximity for three years will start to get on each other's nerves. And then there is hunger: when you've won a league title three times, does the fourth really matter as much as the first?

To suggest Barcelona are in decline is, of course, premature, but the fact remains that they are now five points behind Real Madrid. They beat them impressively enough before Christmas to suggest they remain, by some distance, the better team, but to have failed to win five of eight games away from home this season is more than a blip. Yes, Barça may have been the better team in all those five games, but if shots are raining in on goal and not going in, while there may be an element of bad luck, there must surely also be a suspicion of a lack of precision or focus, an almost infinitesimal slackening that causes the slide from exceptional to merely very good. Certainly on Sunday there was a feeling that, once Cesc Fábregas had given Barcelona the lead after 16 minutes, they eased off, as though they expected the goals simply to flow. That allowed Espanyol into the game so that, by the time Alvaro Vázquez levelled with four minutes remaining, they thoroughly deserved their equaliser.

Yet in many ways, Barcelona are a side set up to endure. Like Ferguson, who reflected last week on how those who have been brought up at a club have more instinctive loyalty, Guardiola has a stock of homegrown talent. The impression is that most players play for Barcelona because they want to rather than because it's a convenient way of paying for the cars and clothes and rounds of Jaegerbombs.

And Guardiola seems, right from the start, to have been aware of the dangers of entropy. He has brought in a couple of new players each season – Alexis Sánchez and Fábregas being the big signings last summer. He has given young players, most notably Thiago Alcântara, their chance. He has been ruthless when he felt he needed to be, as Zlatan Ibrahimovic found out.

Perhaps most interestingly of all, he has fiddled with the formation, his experimentation with a back three giving Barcelona an extra option and allowing him to outmanoeuvre José Mourinho in El Clásico. That flexibility, though, as Sid Lowe has mentioned repeatedly, has come at a cost: Barça don't quite have the same snap or fluidity they have had in the past: learning a new system has disrupted the old.

The tactical change seems to have made Barça potentially better against the best sides (it's hard to read anything into the Champions League groups even if they did play Milan twice, so the only real evidence is El Clásico), but has cost them against mid-ranking domestic sides that last season they swept away. If that is the case, then it may be that, if Barcelona end up winning the Champions League this season, the gamble has been worthwhile: after all, to win a third Champions League in four seasons, to become the first side since Sacchi's Milan to retain the title, would be a greater achievement than winning another league title.

From a personal point of view – although there's nothing to suggest that Guardiola is overly concerned by individual honours – winning a Champions League this season would make Guardiola only the second man, after Bob Paisley, to win Europe's top prize three times. Win it this season, of course, and next season Barcelona might be looking at a first hat-trick since Bayern Munich in 1976, Guardiola could become the first coach ever to win a hat-trick and the first to win the competition four times. The danger, though, is the boost it would give José Mourinho and Real Madrid were they to win the league.

There is a poignant moment in Provided You Don't Kiss Me in which Duncan Hamilton recalls meeting Peter Taylor outside the dressing room in the Bernabéu shortly after Nottingham Forest have beaten Hamburg to retain the European Cup. Taylor, Hamilton said, was like Jay Gatsby, thinking of future glories while realising that the dream was already slipping away. Barcelona seem to be in a similar position now. If they can kick on, if they can overcome Guttmann's three-year rule, then their achievement will be truly historic; if it does all slip away, then they will merely have been another excellent team.