Dutchman Clarence Seedorf believes AC Milan team-mate Ronaldo may not return to playing football after the Brazilian suffered a serious injury this week.
Ronaldo screams in pain after injuring his knee against Livorno
The 31-year-old striker needed surgery on Thursday after rupturing a tendon in his left knee against Livorno.
"He's one of the greatest to play the game and hopefully this is not the way it's going to end," Seedorf told BBC Radio 5 Live.
"I hope he makes the right decision at the right time."
Holland midfielder Seedorf said the moment the Brazil star collapsed he knew it was a serious injury.
"Nobody wants to watch moments like that," said the 31-year-old, who accompanied Ronaldo to hospital.
"My heart stopped beating because it was like watching a repeat of the injury he suffered playing for Inter Milan against Lazio. His reaction was the same."
He added: "It's the third time it has happened to him - the third knee injury he has picked up. That's why I'm thinking it may be the end.
"We'll see what happens over the next six months - he now needs to focus on rehabilitation."
Ronaldo, a three-time Fifa world player of the year, suffered the same injury in his right knee in November 1999.
He lasted seven minutes on his comeback in April 2000 before damaging his knee again, which sidelined him for another 20 months.
I think within a few months he could return to being the champion that we all know him as
Milan owner Silvio Berlusconi
But AC Milan vice-president Adriano Galliani believes the injury is not as bad this time around.
"He is still feeling a little pain but it seems the tendon of the knee is in better shape than that of the right knee which was operated on eight years ago," he said.
"I am not a doctor and therefore I will not be making a guess at how long he will be out.
"He will need to stay in hospital about a week but after that I do not know where he will decide to go - whether he will return to Milan or go to Madrid with his son."
AC Milan owner Silvio Berlusconi remains hopeful that the striker will return to playing.
"I called him and told him to believe in himself. As a 31-year-old, he's very young," he told Italy's RAI TV.
"He has an extraordinary physique and I think within a few months he could return to being the champion that we all know him as."
Milan coach Carlo Ancelotti added: "We're all very sorry and worried about what happened.
"I don't want to say it's the end of his career because only time can decide - all we can do is stand by him."
"He (Ronaldo) is still feeling a little pain but it seems the tendon of the (left) knee is in better shape than that of the right knee which was operated on eight years ago."
Ronaldo, who is out of contract at the end of the season, struggled with his weight at Real Madrid before joining Milan in January 2007 for £4.95m.
And Galliani added that a replacement could be signed in the Brazilian's absence in the summer.
"I am not ruling out bringing in another player but we will have to see this summer," he confirmed. "Our aim this season is to finish fourth in the league, meaning we will qualify for the Champions League."
Milan face Premier League leaders Arsenal in the first leg of their last-16 Champions League tie on Wednesday.