The Real Madrid star had been plagued by injury problems in the closing stages of the 2013-14 campaign, but returned in time for Portugal's 4-0 defeat to Germany in their World Cup opener.
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"Ronaldo didn't leave training early. He finished his training at the same time as the rest of the players," Beto said at a press conference.
"Cristiano is 100 percent fit to play. Every match he starts to play, it's because he's fit to play.
"He loves football but his body is the most important thing for him, to be fit, to be healthy so if he starts the match it's because he's ready, he's fit."
Portugal desperately need a good result against USA after their defeat at the hands of Germany and Beto is confident they can bag the full three points.
"The best version of Portugal can beat the United States. We respect the USA as a very good team.
"They have grown up over the last years so we have to respect them, but we believe in ourselves, we believe in our work, in our quality so we have to be responsible.
"We have to be in the best condition to win this match and we believe it."