Portugal reject long-term fears over Ronaldo
Portugal's forward Cristiano Ronaldo runs during a training session at the team's base camp in Campinas during the 2014 FIFA World Cup football tournament on June 20, 2014 - by Philippe Desmazes
Speculation has been rife over the world player of the year, who is suffering from tendonitis and has repeatedly needed ice treatment during training sessions in Brazil.
On Friday, Portugal's captain had his left knee taped as he took part in light drills at the team's training camp near Sao Paulo, raising expectations he will play Sunday's game in Manaus.
Some reports have questioned whether the Real Madrid forward was risking aggravating his condition by playing at the World Cup. But Postiga said Ronaldo was able to train unimpeded.
"I don't think that's a question to be raised at the moment because he's training with us, he's practising with us and he's doing all the exercises," Postiga told reporters.
"I'm not a doctor but I don't think that's in question."
Ronaldo, 29, caused further concern over his fitness on Wednesday when he stopped training early and had ice applied to his knee at a public session.
On Friday, the Real Madrid forward showed no signs of discomfort as he took part in a warm-up passing exercise with team-mates Nani, Miguel Veloso, Helder Postiga and Ruben Amorim.
Watching media were ushered away after 15 minutes but Ronaldo's appearance raises the likelihood he will play Sunday's game, which has great significance after Portugal's 4-0 loss to Germany.
Reports have been circulating questioning the fitness of Ronaldo, who is considered critical to Portugal's chances of success at the World Cup.
After Real Madrid's win in last month's Champions League final, he missed two Portugal friendlies before returning to action in their final warm-up against Ireland.
Portugal also have injury concerns over goalkeeper Rui Patricio, striker Hugo Almeida and defender Bruno Alves, who are all undergoing treatment.
But Postiga said Ronaldo had been training "at his maximum" ahead of the game against USA, who beat Ghana 2-1 in their Group G opener.
"I personally think that the Cristiano Ronaldo talk is a bit too much so I imagine how he must be feeling about that," Postiga said.
"But regarding his physical condition, he's been training at his maximum and if it's not listed in the clinical information of the Portuguese (football) federation it's because he's fit to play."