The Real Madrid man has been plagued with a troublesome knee problem in recent weeks and is reported to have left Portugal's training early on Wednesday.
Ronaldo was pictured with heavy strapping and an ice pack on his left knee having been under close observation by their medical team.
Portugal's captain was confirmed as suffering from tendonitis in the build-up to the World Cup before being ruled out of their friendly against Mexico earlier this month.
He then played just over an hour against the Republic of Ireland but again limped out of training prior to their opener.
If he has to withdraw, as suggested by some reports, it would represent a major blow to Portugal's chances at the competition.
Paulo Bento's side have suffered a disastrous start to the World Cup - tasting defeat against Germany, having seen Pepe sent off, before losing full-back Fabio Coentrao to a groin injury.
