The world player of the year left the training field on Wednesday early with an ice pack strapped to his left knee, having suffered from knee and thigh injuries since the end of the domestic season that almost ruled him out of Real Madrid's UEFA Champions League final victory.
Ronaldo played during Portugal's 4-0 defeat to Germany on Monday, although reports from Spain claimed Ronaldo ignored doctors’ advice which was to sit out for two months or risk further injury.
Jose Carlos Noronha reportedly said: "Let your knee rest, or your future is in danger", to which Ronaldo replied: "I'm the one that says whether I'm playing or not."
Portugal face Jurgen Klinsmann's USA side on Sunday, where victory for the Americans would put them through to the last 16 for the fourth time in their last seven tournaments.
USA beat Ghana 2-1 in their Group G opener thanks to a late winner by John Brooks, but Klinsmann remains wary that Ronaldo could be in an 'angry' mood should he play on Sunday.
"We need to show Portugal how good we are. Now they are going to come into Manaus pretty angry and I don't now how Ronaldo is going to perform when he is angry", said Klinsmann, who was formerly part of the West Germany side that won the 1990 World Cup.
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