He would not say so, at least not publicly, but José Mourinho probably agreed with his captain. This time, they surely saw eye to eye for once, not least because they had to. "If finishing 25 points behind Barcelona in the league means winning the Champions League, I would sign now," Iker Casillas said, exaggerating for effect. That was in December; two months later it could become a reality, simultaneously seductive and troubling.

Real Madrid are third, 16 points behind Barcelona. This has been a tense, conflictive and unsuccessful season – so far. Mourinho long since declared the league "impossible". There remains a Copa del Rey semi-final, second leg but its importance is relative. What really matters is the Champions League.

Mourinho is a collector, acutely aware of his place in history. Madrid brought him here to do what he did with Internazionale: win the European Cup and defeat Barcelona. He came to do two interrelated things: win the league in a fourth country and win a third European Cup with a different team. No coach had ever done either; do so here and his collection would be unique.

In the first he was successful. Madrid completed a brilliant, record-breaking league campaign last season. Two semi-final appearances, against Barcelona and Bayern Munich, ended in defeat. In the second, he still waits. He may get a second chance elsewhere but this is his last opportunity here. For Madrid, too, it is symbolic: they have waited a decade for the decima, their 10th European Cup. It is starting to feel rather a long time.

Mourinho has not enjoyed Spain. It is hard to see how he can continue beyond the summer, or why he would want to. There have been too many conflicts, bridges burnt; the question is what the exit strategy is. The absence of one may be the only thing that can keep him here, where he has been unable to do what most defined him in England: build a strong, united dressing room and manage the media.

The Madrid-based daily Marca recently claimed that the club captains, Casillas and Sergio Ramos, told the president, Florentino Pérez, that some players would leave if Mourinho continued. Covering the story for Mexican television, a journalist said it was "vox populi" that the dressing room does not get on with Mourinho, which might not have mattered if the journalist was not Casillas's girlfriend. Her remarks, widely criticised, confirmed the tension that had already emerged but many, including Pérez, had tried to deny.

The goalkeeper's relationship with the coach, like that of a handful of players, is uneasy. Much of the media has turned on Mourinho too, often viciously and indiscriminately so – and at least in part because they have felt "freed" to do so. And, although he has unconditional supporters, Mourinho has been whistled at the Santiago Bernabéu.

They will not whistle tomorrow. The European Cup is too important – enough to unite, to subsume all in the search for success. The absence of Casillas with a hand injury may even be beneficial, removing a focal point of tension, an alternative source of power. And for all the domestic problems few doubt that this side is good enough to win the tournament. Most assume they are better than Manchester United.

And if they do win it, everything changes. Lose and Pandora's Box opens; win and redemption and vindication could be the reward alongside the greatest trophy of all. It would also provide the conditions for club and coach to part company without rupture or loss of face.

It comes down to one hand but it is a hell of a hand. This competition will define this season and probably Mourinho's period in Spain. From the start of this season, the Champions League was the priority; now, it is everything. But then, in a way, it always has been: this is the trophy that defines Madrid, the one on which their identity is constructed. In the Veterans' Association only nine team photos adorn the wall: European Cup winners, enjoying the incontrovertible immortality Mourinho seeks.

There is a familiarity about the situation too, comfort in the knowledge that league struggles are not necessarily taken into Europe. Quite the opposite. When Madrid won their seventh, eighth and ninth European Cups, they finished fourth, fifth and third in the league, never close to the title. "It was as if we pressed the 'off' button in the league," Raúl recalls of the seventh; Lorenzo Sanz, the president then, describes their domestic season as a "via crucis"‚ a year of conflict and defeat. Jupp Heynckes was sacked at the season's end and no one cared. He departed a European champion.

Madrid did not win the league when they clinched their sixth European Cup in 1966, either. And even when they won five successive European Cups, forging their legend as the greatest side ever, back home it was different: of 10 trophies between 1956 and 1960, Madrid won two, the league in 1956-57 and 1957-58. The European Cup, though, eclipses all else. It was true then and it is true now.