Ronaldo arrived at the FIFA World Cup with an injury cloud hanging over his head with the Real Madrid forward struggling with knee and groin problems.
Although Ronaldo played over an hour in Portugal's final pre-tournament friendly and the 90 minutes of his team's 4-0 loss to Germany in their opening World Cup fixture, the regular sight of the 29-year-old with strapping or ice on his left knee has ensured speculation about his fitness has not gone away.
But Postiga insists Ronaldo will play against the United States on Sunday.
"Cristiano was there during the whole training session," Postiga said on Friday.
"Regarding the ice on his knee if you have a look around after the training session you will see that a dozen of players will be using bags of ice. I use three bags of ice myself. So I can't see this as a reason for concern.
"Cristiano is training and playing well and will help us. That's our focus now. Using ice after a training session is something really normal."
Portugal coach Paulo Bento has had to deal with a relatively long injury list in Brazil with Ronaldo, Raul Meireles and Pepe arriving at the World Cup with fitness issues, while Fabio Coentrao (groin), Hugo Almeida (thigh) and Rui Patricio (thigh) all injured themselves against Germany.
Bruno Alves missed Friday's training session as well, reportedly due to a thigh injury, and while Postiga admitted Portugal have been unlucky, the 31-year-old Valencia striker dismissed suggestions of any underlying problem in the squad.
"We've been suffering with lots of injuries indeed. Hopefully this situation will improve. But there is no special reason for that. This is something normal," he said.
"We all had long seasons and maybe this is the logical explanation for it. It was a tough season for us. Hopefully everything will be better until the end of the tournament and no other players will suffer with injuries."
