Could this be the end of the great Barcelona team of Messi, Xavi and Iniesta?

Madrid's press certainly think it could be. "Fin de ciclo" screamed the headline on the front page of AS, one of Madrid's two daily sports newspapers, last Wednesday after Barca's 4-0 caning at the hands of Bayern Munich.

The two sides meet again tonight in the second leg of their Champions League semi-final. Surely the gods won't grant another reprieve to Barca, of the kind it enjoyed against Milan in March in the tournament's Round of 16, when Messi & Co smashed four goals past the Italians at Camp Nou to proceed 4-2 on aggregate.

This time the difference is four goals, not two, going into the game; no team has overturned that margin of deficit in the competition in 171 tries since the away goals rule was introduced. If Bayern score – it's been more than a year since they failed to score on the road – Barca will need to get six.

Bayern, of course, have been in imperious form this season. It is a team on the cusp of greatness. What, one wonders, could it be like once Toni Kroos and Holger Badstuber are back from injury, and when Dortmund's Mario Götze and, possibly, Robert Lewandowski are added to its roster?

Last week, they wrapped up the domestic league title with six weeks to run in the Bundesliga, which is a record, and they are driven by a need to erase the embarrassing defeat in their own home by Chelsea, whom they dominated, in last year's final. Will it be cagey against Barca, and try to preserve their lead? More likely they will smell blood.

There have been signs of decay in the Barca edifice for a while now. Part of it is inevitable, the result of an understandable lack of hunger, of burnout, after amassing 14 trophies in four seasons, notwithstanding international adventures. Seven of the starting XI in Spain's World Cup-winning team in 2010 were drawn from their ranks, six when they retained the European Championships crown last summer.

The mindless, long-distance friendlies that Spain's international players have been forced to play over the last few years by its cash-hungry football federation has not helped. This season alone their star players have been packed off for games from La Roja to Pontevedra, Puerto Rico, Panama City and Doha.

It is significant that Messi picked up his much-publicised right hamstring injury – it is said he was "hamstrung" during the first leg match with Bayern – against Paris Saint-Germain four weeks ago after returning from an international with Argentina at altitude in Bolivia.

Guardiola saw signs of rot setting in, such as unruly socialising by Gerard Pique and Dani Alves, two players he dropped last season; he had also run out of gas himself.

The club has been bedevilled by bad luck over the last year. There have been injuries, notably a succession to the 35-year-old body of club captain Carles Puyol.

Cancer returned to Eric Abidal, arguably the club's best defender, certainly the one who makes least mistakes, keeping him out of action for 12 months at a time and Barca have been leaking goals. Fifteen times this season they have come from behind to win games, registering their worst defensive record since 1962 a couple of months ago.

Cancer also hit manager Tito Vilanova again. Questions have been raised about Vilanova's future at the club. He says he will stay on, addressing the naysayers in his first press conference in three months last Friday. His inertia during the rout by Bayern last week was troubling. His only intervention was to put on David Villa with 83 minutes on the clock. Perhaps he was overcome by shock. It is no mitigation. As Gary Lineker put it wryly: "Only one of these sides needs Pep Guardiola - and it's not Bayern."

The pressure on managers at Barcelona is not normal. Johan Cruyff, the club's longest-serving coach since the Second World War, suffered a heart attack and never managed a club again after his stint in charge. Frank Rijkaard, Guardiola's predecessor, moved out of his family home into a hotel close to Camp Nou during his last chaotic season.

The club's internal politics over the last 30 years have been Shakespearean, full of vicious personality clashes amongst its presidents and directors. Despite the club's huge wealth (it rakes in about €½bn a year, almost €100m more than, say, Manchester United), it's never far from implosion. Even though it won nine league titles in the last 20 seasons, it went three trophyless seasons after the fallout and bickering that resulted from Luis Figo's move to Real Madrid in 2000.

Vilanova needs to recharge a team that is overly dependent on Messi. The graph of his contribution to Barca's goals over the last five years is striking: 2008-09 (32%); 2009-10 (43%); 2010-11 (46%); 2011-12 (56%); and 2012-13 (60%). On Saturday, Barca trailed Athletic Bilbao 1-0 when he was sprung from the bench after an hour. Within minutes, he weaved his through three players on the edge of the box, reminiscent of George Best's memorable goal for the San Jose Earthquakes in 1981, except in a tighter space, before passing the ball into the net.

With him in the side, optimism will never be far away for Barca fans. He's only 25 years of age, the same age as Cesc Fabregas. Jordi Alba and Sergio Busquets are 24, Pedro and Pique 26; and Iniesta is 28. Add to their number players such as Neymar, whom Sport newspaper claims Barca have paid a €10m deposit, and Thiago Silva or Mats Hummels, names touted over the weekend in the press, and rejuvenation is at hand, just not in time for tonight's clash against Bayern Munich.

Richard Fitzpatrick is the author of El Clásico: Barcelona v Real Madrid, Football's Greatest Rivalry, published by Bloomsbury