On the eve of Barcelona's Champions League tie with Manchester City, Javier Mascherano was called on to deliver a message. Barcelona had lost three of their past six La Liga games and had just been defeated at relegation-threatened Valladolid, their lead at the top slipping through their fingers. Crisis loomed but the Argentinian looked at his inquisitors and insisted: "It's March." Then came the plea. "Wait for us to be dying before you kill us off! We're still alive. Don't kill us yet!"

How about now, Javier? It's April now. On Wednesday Barcelona face Real Madrid in the final of the Copa del Rey. There is excitement but above all there is fear; a sense that, teetering on the edge, here come their biggest rivals, ready to give them the final gentle, gleeful push into the abyss below. Three, two, one ... none? Could Barcelona lose a potential treble in record time?

Back then Mascherano said that league, Champions League and Copa del Rey were all on. Momentarily, he appeared to be right: Barcelona beat City and went to the Santiago Bernabéu where they beat Real Madrid 4-3. Three days later, Madrid were defeated at Sevilla; Barcelona had the title in their own hands. Maybe he had been right when he said: "In pre-season you dream of being in a situation like this. It's time to step on the accelerator."

Instead, Barcelona have been a car crash. It is six days since they were beaten by Atlético Madrid in the Champions League, the first time they have not reached the semi-final in seven years. The criticism was intense and their coach, Gerardo Martino, did himself few favours, particularly when he made the baffling admission "we were not interested in Messi being involved that often", but there was much to fight for. Three days later the headline on the front of the pro-Barcelona newspaper Sport declared: "We'll die for the double." The other Barcelona-based sports daily El Mundo Deportivo added: "Two to go."

Make that one. Barcelona lost 1-0 at Granada, a team fighting relegation. This time the headline pointed the finger at Martino and at the impending crisis. "Tataclysm," it said.

When the players returned to the Camp Nou from Granada, there were a handful of fans waiting. Some shouted abuse. Messi was criticised too; the man who has often disguised the crumbling edifice, whose performances could be undermined by the continued failure to sign an actual defender, stands accused of reserving himself for the World Cup. Knives were being sharpened across Catalonia.

Defeat left Barça third, a point behind Real Madrid and four behind Atlético with five games left. Their chances of winning the league appear to have vanished.

And, now, make that none? Three days later they face Real Madrid in the cup final. Less an opportunity, more an obligation.

The optimists reason that they are the side who have faced Real Madrid twice this season and beaten them both times, including that stunning win at the Bernabéu. The optimists note that Leo Messi, the focus of much of the criticism, has scored 21 times against Real Madrid in 27 games, more goals than against anyone else. They will note, too, that Cristiano Ronaldo, who scored the winner last time they met in the final at Mestalla in 2011, will miss the game through injury but the optimists are outnumbered.

Víctor Valdés is injured and there may be no natural centre-backs available. Carles Puyol, Gerard Piqué and Marc Bartra all travelled with the squad but none have been given the all clear by doctors. Mascherano's performances have dipped markedly.

And the problems go far deeper than that. All year there have been doubts, played out against a backdrop of institutional collapse; there has been bad luck but the planning has been appalling and the price being paid is a high one.

As Andrés Iniesta put it, win or lose: "If there have to be changes there have to be changes." Two of the three captains, Puyol and Valdés, have already announced their departure but other movement – and there was to be lots of it – is on hold, pending the transfer ban imposed by Fifa for breaking rules on signing international players under 18. It is already overdue.

All season Barcelona have been just a couple of defeats from a crisis. This is not just a snap reaction to two sudden, shock defeats and the worst thing about those losses is that they were coming.

Against Atlético, only the two goalkeepers ran less than Messi and only just: José Pinto ran 1.5km fewer. In five games against Atlético, Barcelona have won none. They have scored just twice. They have now gone two matches in a row without scoring and if that does not sound too bad, look a little deeper: they have scored only five goals in their last five games, against Granada, Betis, Espanyol and in the two games against Atlético. Three came from penalties (one on the rebound) and another was an own goal.

Some see redemption in the cup but success at the Mestalla may bring mixed emotions. Charly Rexach played for Barcelona between 1965 and 1981 and talks about winning the cup, "their cup", as a way of "really pissing [Real Madrid] off." It was also a way of salvaging a season at a time when Barcelona knew they were inferior: the league title in 1974 was their first in 14 years and they had to wait a further 11 to win it again under Terry Venables.

This is nowhere near as dramatic, of course, but there is a desire not to return to that identity. This century, Barcelona are Spain's most successful club, not Real Madrid. Victory in the Copa del Rey will be celebrated but a cup success, even against their greatest rivals, will not make this a good season and will not alter the need for repairs. It will help, though.