According to reports, Ronaldo limped out of Portugal's training session with an ice-pack on his left knee. The Real Madrid forward had trained for about 20 minutes before going off.
Ronaldo, one of the star attractions at this year's World Cup in Brazil, had already missed two of Portugal's friendlies before the tournament due to knee and thigh problems.
The 29-year-old Portugal captain is aiming to win his first World Cup title in Brazil. After leaving the training session, Ronaldo watched his teammates at work, from the bench, "BBC" reported.
Ronaldo's team-mate Joao Moutinh was quoted as saying by "BBC", "Ever since he started to train with us he has been 100% like the rest of us.
"Everything is fine with him but there is some caution, not only with him. Many other players were using ice as well. There is nothing to worry about. The ice is normal after training."
Portugal are in Group G with Germany, Ghana and USA.
World Cup commenced yesterday (June 12) with hosts Brazil defeating Croatia 3-1 in Sao Paulo.