The 29-year-old has been struggling with a muscle problem over the past two weeks and was rested for the Seleccao's friendly clash with Greece in a bid to aid his recovery.
However, the Portuguese FA has since revealed that Ronaldo has also been struck with "left leg patellar tendonitis" and took part in a separate running program in training early this week, with Pepe, Raul Meireles and Beto also battling to recover full fitness.
The news comes as a blow to head coach Paulo Bento, who watched his side labor to a 0-0 draw against Greece last week in the absence of their record goalscorer.
However, Ronaldo's former Manchester United teammate Nani - who was named in Bento's final 23-man squad for the World Cup - insists the reigning Ballon d'Or winner is calm about his hopes of making it to Brazil.
"I don't believe the situation with Ronaldo is worrying," he said on Tuesday following a training session in New York. "He's going well, and he is relaxed about it ... things are going the right way.
"We hope that Cristiano is in good enough shape to play - and to help the team."
Nani also stressed that the squad as a whole is more than capable of stepping up to deliver in Ronaldo's absence as they prepare to take on Germany in their Group G opener on June 16.
"If for some reason (Ronaldo) is not there, whoever is there playing has to be confident in their ability to get the job done," he said.
"The World Cup is a one-of-a-kind challenge. I want to be fully fit, and in top form alongside my team-mates. I could not be there last time, but happily, I am here now.
"The players are happy, confident and relaxed, we have been working well. I hope that is the basis for a good campaign."
Portugal faces Mexico on Saturday in its latest warmup fixture.