At one end, the man who came to replace Iker Casillas in the short term; at the other, the man earmarked to replace him in the long term. Both Sir Alex Ferguson and José Mourinho were quick to describe this as the biggest game in Europe, the tie of the round. Both men field Spanish goalkeepers very much in the spotlight, men under pressure: Diego López and David de Gea. Men whose paths have crossed.

When Casillas broke a finger, Madrid acted quickly to replace their captain. They turned to a former youth team player who had long since moved on: López. At the time he was sitting on the bench at Sevilla. At 31, it is a short-term solution. In the long term, one of the names that has been floated is that of the man many have earmarked to replace Casillas in the national team too. De Gea is back in Spain on Wednesday night. Reports in Spain have speculated over a permanent return.

As for López, he is back at Madrid. He could have ended up in Manchester himself: when Edwin van der Sar neared retirement, he was among the men scouted as a replacement. He was Spain's No3, behind Casillas and Pepe Reina. Now, unexpectedly, he is Real Madrid's No1. A reliable understudy, swiftly purchased from Sevilla for €3.5m (£3m), and put straight into the Real Madrid side.

So far, so good. Against Barcelona in the Copa del Rey, López kept Madrid in the game. Serene and sensible, he has quickly settled. But it is still early and should he falter, they will be waiting for him. And for Mourinho. There is no avoiding the status of the man he has replaced: Casillas is the most successful international captain of all time, an institution at Madrid. When Mourinho dropped him, media and supporters turned on the manager.

"Whenever Casillas is missing, there's something missing," De Gea said here. "He's the captain of the national team and Real Madrid. Of course his absence is important, a great loss for them, but they have a very good goalkeeper who I am sure will do very well." De Gea speaks from experience; he knows both men. He also knows that, in the long term, there are many in Spain who aspire to bring him back. Madrid are not alone: Atlético Madrid, conscious that they will lose Thibaut Courtois, would love to have him back. And Víctor Valdés has announced that he will not continue at Barcelona.

De Gea's difficulties at Old Trafford have given rise to such suggestions; it is hard to avoid the sensation that England has not been exactly what he hoped. A different game, one in which he has expressed surprise at just how much opponents are allowed to seek him out physically. A different environment, too.

As he told The Guardian in the summer: "At times people don't realise that it's not easy. When you come home in Spain, your family are waiting for you. When you're in England, they're not. For a Spaniard it's different. The worst things? It rains a lot, it's cloudy, it gets dark very early, maybe you're a bit bored. The winter can be hard. You miss friends and family. People don't always appreciate that. There are hard moments."

He has not been exactly what they hoped either. At least not yet. But those that know him insist that if there is one thing that characterises De Gea, it is that nerves do not follow mistakes. He looks timid but he is tough.

Abel Resino, the former Atlético coach recalls the days before his debut: "I spoke to him about it. 'Mister,' he said to me, 'I'm better than all the other goalkeepers here. I'll get in the first team here.' There was something I liked about him: there was assuredness, security. It wasn't arrogance, it was conviction."

De Gea is young; he needed time. And in the meantime, he would not be sunk. Wednesday night is the perfect stage, albeit one on which he has previously suffered at Madrid's hands. Both the goalkeeper and the coach were quick to suggest that now he is ready.

"When you get to a new club and a new country, there are bound to be complicated moments," De Gea said. "But you need that to get stronger and improve. I'm a better goalkeeper than I was when I went to United."

Sitting next to him was Ferguson. "The boy has shown his character," the United manager said. "He has taken a lot of criticism and a lot of it unfair. What we are good at [at United] is developing people. [He was] a young man, very young, not used to the English game, who couldn't speak the language and still had to mature physically. But it's a like a young kid taking his first steps forward. He wobbles, gets up, wobbles, gets up again and eventually he walks. The boy is walking now."