The banner at one end of Camp Nou declared "Tornarem A Vèncer". We Will Win Again. They had brought it out for the clásico against Real Madrid on Saturday night but it is even more pertinent now. Barcelona were beaten by Real, José Mourinho winning at the Camp Nou at last. It was the first time that Barcelona had been defeated at home since September 2010 and only the third time they have lost two consecutive games under Pep Guardiola. Under him they have never lost three times in a row.

One of those two-game losing runs was at the start of Guardiola's time in charge, back in 2008, and that season ended with a treble; it also ended with two more consecutive defeats - with reserves and <italics>after</italics> the league title had been clinched. This is an uncomfortably unfamiliar sensation and the stakes now are higher. Against Chelsea Barça must win again. By losing to Madrid the Catalans relinquished the league title four years later; lose to Chelsea and they will relinquish the Champions League title. In four days their season risks unravelling. Yet it could also look like becoming a great season once more – "historic" in the words of Xavi Hernández.

In London, there was optimism in disappointment. Barcelona's players analysed the performance more than the result. There had, they judged, been sufficient chances to beat Chelsea and there would be again. Back in Barcelona, defeated by Madrid three days later, the feeling was different. Through the gloom, though, came hope.

Guardiola interrupted the first question after the clásico to say: "First of all, congratulations to Real Madrid for the game ... and the league." There was resignation in his voice, a quiet monotone. He looked as tired and weary as his team had. The damage to Barcelona's title chances was permanent; the question now was whether the damage to their European hopes would be too. Continue in the Champions League and there is an opportunity for redemption and maybe for revenge in a possible final with Real. Europe eclipses all else. Go out, and the sense of decline, if exaggerated, will be unavoidable.

Europe takes on even greater significance now. As the headline in one of the Catalan sports dailies put it the morning after the clásico: "Goodbye league; hello Champions League." The Barcelona coach knows it will be difficult: he described Chelsea and Real Madrid as two of the best defensive teams in the world. In two matches against them Barça scored once. Leo Messi had not found the net at all. The feeling of impotence was concerning. Guardiola noted the evident fatigue and admitted he had not spoken to his players; that was a task he left until Sunday when, with only two days to prepare for Chelsea, they reconvened.

"The players are sad [in the dressing room] and there is no consolation in words," he said. "We have not stopped being what we are but one of the tournaments for which we had fought long and hard has escaped us. These boys have been playing for a long time, working since August. You ask for the same brilliance as ever but there are times when, after playing for everything for four years, it's hard. I hoped that wouldn't come against Madrid."

It did. The risk is that it continues against Chelsea. "We'll see on Tuesday whether we pay for this," Guardiola said. "I think the players will lift themselves up. They have been through moments like this before and got back up again and carried on. They know how important the game is and Chelsea are a very physical team. If we go through, the focus will be on the two finals, and if not we will look to the Copa del Rey final."

There was a message in there. The temptation in the wake of the clásico, and with Chelsea now redrawn as a threat rather than an opportunity, is to predict Barcelona's eclipse, to foresee a season that ends empty‑handed and brings an era to a close. In the buildup to the clásico, the coach was keen to stress that his team had "already won": in sport losing is normal and Barcelona's success resided in the fact that they had competed to the end yet again – on all fronts, too. Five European Cup semi-finals in a row is something of which Guardiola says he is "especially proud".

Besides, a team that had won 13 of the last 16 trophies available deserves the benefit of the doubt. And talk of ending this campaign empty-handed misses two points: firstly, Barcelona have won three trophies already this season: the European Super Cup, the Spanish Super Cup (against Madrid) and the World Club Championships. And secondly, it could yet be 15 of 19. Barcelona have the final of the Copa del Rey to come, having defeated Madrid on route. More importantly, they still have the Champions League semi‑final second leg.

"This is not a delicate moment, it's a moment of hope," Xavi said. "The chance to complete a great season is still in our hands. We have won three trophies. It would be historic to win the Copa del Rey and reach another Champions League final." If they are to reach Munich, only one result will do now: Barcelona must win again.