Portugal star striker Cristiano Ronaldo is reportedly out for the remainder of the World Cup, as he continues to struggle in staying fit because of a bothering knee injury he sustained in the buildup towards the tournament in Brazil, according to latest rumours in the internet.

The 29-year old Ronaldo is among the biggest stars in the ongoing FIFA World Cup in Brazil, but dealing with a serious knee injury might be too risky for his long-term career.

According to the Spanish publication El Confidential, renowned Portuguese surgeon Jose Carlos Noronha is raising a red flag on Ronaldo's injury, as he advised him to pull out of the World Cup or suffer a dire consequence of sustaining an irreparable damage on his bummed knee.

"NBC Sports, citing a story in El Confidential, is reporting that Ronaldo's doctor wants him to pull out of the World Cup due to a knee injury. Portuguese surgeon Jose Carlos Noronha is advising the Real Madrid star that he could risk tearing the tendon in his knee beyond repair," via TSN Canada.

The Daily Mail UK reported that Ronaldo struggled to be in fine footing during his latest training session, which saw the Portuguese striker limping on the field.

Via Daily Mail UK.

"Ronaldo declared himself '100 per cent' fit on the eve of Portugal's opening 4-0 demolition at the hands of Germany but, although he managed to play the entire game, he was largely peripheral and failed to change the course of the result.

It now appears that the Ballon d'Or winner has suffered a recurrence of his knee problems after receiving treatment during a training session in Campinas.

Ronaldo was taken through a series of stretches before having his left knee bandaged with an ice pack and helped off the field 30minutes before training finished. Ronaldo watched the rest of the session from the sidelines."

If the story holds true, Ronaldo's absence is not only a big blow to the no.4 ranked Portuguese national team but the entire tournament as well, as World Cup organizers bank its marketability on the star player of each participating squad.

Portugal, who already lost Real Madrid defender Fabio Coentrao in the loss to Germany, is scheduled to take on USA on June 22 and then Ghana on June 27.