According to reports, Ronaldo has been forced to train in isolation away from the rest of the Portuguese squad nursing two niggling injuries.
A statement from the Portuguese FA today confirmed that Ronaldo was suffering from "left-leg patellar tendinitis” whilst reports in The Daily Mail suggest that the Real Madrid man is also carrying a muscular injury in his thigh.
Portugal are due to face Mexico in their next warm-up match on Saturday, ahead of opening their World Cup campaign in Group G against tough opponents Germany on June 16.
Without Cristiano Ronaldo, Portugal would struggle to offer a goal-scoring threat. Striking options in the Portuguese national pool seem to have dried up somewhat since the glory days of Nuno Gomes and Pauleta, and you only have to look back as far as the sensational play-off hat-trick scored by Ronaldo against Sweden securing a place in Brazil to see how valuable the 29-year-old really is.
It's been a long gruelling season for Cristiano Ronaldo, playing over 50 games for club and country and it's no wonder that minor injuries are beginning to catch up with him.
Full recovery time from tendinitis can be as long as six weeks, but it's unlikely that Ronaldo will face anything like that kind of lay-off.
Former Manchester United and fellow Portuguese international Nani has done his best to quash the fears, insisting he is unconcerned and believing Ronaldo will be fit;
Nani said: “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.”
A possible scare for Portuguese hopes, and a dreadful shame for the World Cup as a spectacle if Ronaldo is forced to sit out.