So how do you stop him? That is what Sir Alex Ferguson has to ask himself as he boards the flight to Madrid.
On the basis that we are talking about someone who has rattled in 20 hat-tricks on the way to 182 goals for Real Madrid in 179 appearances, the problem for United is that this is about the closest thing there is in football to asking how to nail a jelly to the wall. Ronaldo, one suspects, will cherish the opportunity to remind English football what it is missing and, before we even look at the possible tactics that United may employ, one thing is very clear: Rafael da Silva, on the right side of defence, may have to play the game of his life. He may have to do it twice, in fact.
Is he up to the task? United should be encouraged that the Brazilian is no longer the rash and impetuous player of old, still raw occasionally and not fully educated yet in the art of defence but also possessing the whippet-like speed and perseverance that make him a hard opponent to shake off. All the same it is difficult not to think Ronaldo-versus-Rafael represents a significant risk for Ferguson's side unless they have other measures in place – and a clear plan about how to tackle the player they helped nurture into such a devastating destroyer of defences.
They could learn, for example, from the way Jürgen Klopp set up his Borussia Dortmund team when they played Madrid in the group stages. Klopp knew that Ronaldo potentially had the beating of his team, so he worked out that the best way to stop him was to prevent him getting the ball as often as he was used to. And Madrid-watchers should be fully acquainted with the killer pass of choice at the Bernabéu: Xabi Alonso, in his own half, playing the ball right to left, medium to long distance, leaving Ronaldo free against the full-back.
"Our plan was to take Xabi Alonso out of the game," Klopp explains. "If Alonso plays the way he can play, it's almost impossible to defend against Real Madrid."
Klopp instructed Mario Götze to combine his usual attacking instincts with sticking close to Alonso in the space between the penalty area and centre circle where Madrid's playmaker is so effective. "Once we put Götze on him we knew that, if our two full-backs were very mobile, we would have the advantage over Ronaldo. If you block Xabi Alonso it means Pepe always has the ball – and that's a very different thing."
A lot of the debate since Sunday, when Ferguson used Phil Jones as a man-marker to nullify the threat of Everton's Marouane Fellaini, has been about whether the same player could be deployed that way against Ronaldo. What Klopp is effectively saying is that it would be more effective to put someone on Alonso and deprive Ronaldo of his best source. Dortmund drew 2-2 in the Bernabéu, denied a win only by Mesut Ozil's 89th-minute goal. Ronaldo did score in the Westfalenstadion but Dortmund won 2-1. For the most part the tactic worked brilliantly.
Rafael, at 22, is certainly a more rounded defender than he was three years ago, when his red card against Bayern Munich was a considerable factor in United's elimination at the quarter-final stage. If there is a weakness, it is his positional sense or the occasional lapse of concentration. Rafael has, nevertheless, probably been United's best player this season behind Robin van Persie and Michael Carrick and, though he rates Ronaldo as "the complete player", there is one thing in the Brazilian's favour. "I know what he can do because I've played and trained with him a lot. He's not the type of opponent who you would say he is good but I don't know what he can do. I know what he can do."
Ferguson might also have picked up on the recurring pattern in the matches Madrid have lost 1-0 against Sevilla, Real Betis and Granada this season. On each occasion Madrid's opponents have effectively allowed them to have the lion's share of the ball, flooded their own half to reduce the space around Ronaldo and then broken swiftly on the counterattack. It has taken a while for Spanish teams to work it out but the formula against Madrid – unless it is Barcelona, who change their system for nobody – appears to be to play deep, defend with great resilience and commitment, crowd out Ronaldo and then hope, perhaps, for a bit of good luck. "With Madrid you have to give them the ball but deny them the space," Pepe Mel, the Betis coach, said.
The danger here is that Madrid do not keep the ball with the same refinement as Barcelona and that can sometimes encourage opponents to come out of defence and play with more adventure. It is almost always a mistake. Ronaldo is rarely more dangerous than when the opposition are attacking, frequently scoring within a few seconds of Madrid defending a corner.
The difficult part, of course, is putting all this in operation. Ronaldo has a licence to roam that means he will not just stick rigidly to the left if he is finding Rafael difficult to elude. He will simply look for someone else to prey upon. "Ronaldo is a bully," Gary Neville, his former United colleague, says. "He bullies the weakest defender. He does it all the time. He is a monster. You literally need to have two men against him. Even then they might both get beaten."
Rio Ferdinand, asked about the Ronaldo factor, talked of United needing to show "experience, courage and ability … be compact and work hard". Failing that, he said, United might just have to hope he has a rare off-night. "It's obviously going to be an emotional night for Ronaldo and a great night for him, too, but hopefully it gets too emotional for him."
It can happen. It sounds like clutching at straws, admittedly, but Patrice Evra has also made the same point. "It will be emotional for Ronnie because he loves Manchester United. I hope the emotion will get to him."
The alternative view is that Ronaldo may be inspired by the opportunity to be the star of the show. Ronaldo is not usually someone who finds the sense of occasion too much for him. "The best player in the world," Rafael says. Stopping him will be the biggest assignment in the Brazilian's professional life.
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image: © Jan S0L0