The thrashing was the biggest ever loss the Portuguese have suffered at a World Cup and their misery deepened as central defender Pepe was sent off while Ronaldo struggled for form as he battled a knee injury.
The world player of the year will need to be back to his best for the clash with the Americans, who surprisingly beat Ghana 2-1 in their opening game.
Rumours about Ronaldo's condition have been swirling across the internet and officials were forced to deny reports that his personal doctor had advised him to quit the tournament.
A few days' rest seem to have helped and his team mate Miguel Veloso said the player appeared to be in good shape.
"I'm not a doctor but from what I see, Ronaldo is fine," Veloso told reporters after a training session in Campinas.
"He is training, he is jumping and he is shooting. There is not much else to say."
Ronaldo, who was suffering from tendinosis in his left knee several weeks before the 32-nation tournament, was photographed walking out of Wednesday's training session early with an ice pack on his left knee.
Also of concern to coach Paolo Bento is the need to find the right replacements for the suspended Pepe and injured left back Fabio Coentrao whose involvement in the tournament is over.
Goalkeeper Rui Patricio and striker Hugo Almeida were also nursing knocks but have been ruled fit to play, although the latter could be dropped for Helder Postiga after a lacklustre performance against Germany.
INSPIRED DISPLAY
The United States will hope to reproduce the inspired display from the 2002 World Cup when they beat Portugal 3-2 in the group stage, a win which helped them advance to the last 16 at the expense of their rivals.
Coach Juergen Klinsmann, who was in charge of his native Germany when they beat Portugal 3-1 in the 2006 tournament's third-place match, said he expected Ronaldo to fire on all cylinders in the hot and humid city of Manaus.
Klinsmann also hopes that his walking wounded recover, in particular Clint Dempsey who broke his nose after taking a boot to the face against Ghana shortly after scoring inside a minute.
"Now (Portugal) are going to come into Manaus pretty angry and I don't know how Ronaldo is going to perform when he is angry. We need to show Portugal how good we are," he said.
Klinsmann might ponder whether to start John Brooks, who scored the winner against the Ghanaians on his World Cup debut after coming on as a substitute for Matt Besler, while striker Aron Johansson is set to replace the injured Jozy Altidore.
"You have got to deal with injuries," the 49-year old former international striker said. "It will happen more often in this World Cup. Players need to be ready."
