The Real Madrid star left his team's session an hour before it ended Wednesday, sparking fresh fears about his fitness.
The 2013 Ballon d’Or winner’s fitness has been a constant worry for boss Paulo Bento, with the 29-year-old battling tendinosis in his knee as well as a long-standing thigh complaint.
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Ronaldo was a slight doubt for his country’s World Cup 2014 opener against Germany on Monday but managed the full 90 minutes as his side was hammered 4-0 in Salvador.
Bento is now hoping his captain will be fit for what has become a crunch Group G clash with USA on Sunday.
Portugal’s all-time leading scorer could now be a doubt, however, after spending the final hour of his team’s session at Ponte Preta’s Estadio Moises Lucarelli in Sao Paulo with an ice pack pressed to his knee.
He has regularly trained apart from the group in order not to further aggravate his knee problems, and Portugal fans will hope the decision to end training early was merely a precautionary measure.