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How will the USA mark superstar Cristiano Ronaldo?

RECIFE, Brazil -- Former U.S. international goalkeeper Kasey Keller was asked if Cristiano Ronaldo had ever scored on him. His response?

"Probably," he joked.

Keller's memory then kicked into gear, and he recalled how Ronaldo scored against him during his brief loan stint with Southampton back in 2004. Keller insisted that as a goalkeeper, he treated Ronaldo like he would any other player.

"You have to," he said. "Otherwise you'll just get yourself into trouble."

As for his teammates out in the field, that approach is very different.

"I think some of it is just complete concentration," he said just prior to the Italy-Costa Rica match in Recife. "If you're that right-sided back or midfielder, you have to understand that he wants to come inside and shoot. And if you can force him wide, then great. But you're almost forgetting any other responsibilities, and just completely concentrating on him, because if you don't he's going to be past you."

Marking Cristiano Ronaldo won't be easy but if the USA's midfield and defense are in sync, they can make things difficult for him.

The U.S. is set to square off against Portugal and Ronaldo on Sunday in Manaus, and the Americans have already gotten a taste of what can happen when they switch off for even a second defensively. Last Monday, Ghana midfielder Andre Ayew played a combination with Asamoah Gyan, and quickly passed by U.S. defender Fabian Johnson, allowing him to score the Black Stars' equalizer.

"Johnson had a great game, but there was one lapse in concentration and the pace was past him and he just couldn't recover," said Keller. "With Cristiano, you have to double that concentration level."

Stopping Ronaldo is complicated by the fact that he'll float all over the field to find space, meaning if he finds he's having little success against the likes of Johnson, he'll move centrally and see how he fares against a team's center backs.

Of course, there have been numerous questions about Ronaldo's fitness. The Real Madrid star has been battling tendonitis in his left knee, and looked far short of his best in his side's 4-0 thrashing at the hands of Germany, though his teammates didn't give him much help. That said, Ronaldo's physical state could play a huge part in his overall effectiveness.

"Cristiano at 80 percent is still a very good player, but Cristiano at 100 percent is a whole other ballgame," said Keller.

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The other way to stop Ronaldo is to put the clamps on players like Joao Moutinho and Raul Meireles. Moutinho in particular can be lethal in transition if given time and space on the ball.

Of course, the best antidote to Ronaldo is to keep possession, something the U.S. found extremely difficult to do against Ghana, as the Americans completed just 66 percent of their passes. Granted, some of this was down to the psychology of the match, in which the U.S. was taking a safety first approach and clearing the ball out of danger instead of looking to pass. But there is universal agreement that against Portugal, the Americans will need to improve in this area.

"There's a possibility that the U.S. won't have more of the ball than Portugal," said Keller. "But you can't give away possession as easily as they did against Ghana, and just let Cristiano be bombing free kicks and doing different things throughout the match."

Otherwise, it could make for a very long day in the steamy conditions of Manaus.

Jeff Carlisle covers MLS and the U.S. national team for ESPN FC. Follow him on Twitter @JeffreyCarlisle.