The AC Milan striker, 33, fell to the ground holding his knee three minutes after coming on in the second half of Milan's 1-1 Italian league draw with Livorno.
He was in tears as he was stretchered off the pitch amid speculation that he could be out for up to one year - or even face the end of his career.
Milan chief medical officer Jean-Pierre Meerssemann said that tests done in the night "confirmed the total rupture of the rotula tendon in the left knee.
"Ronaldo himself asked to be operated in Paris by the team of doctor (Gerard) Saillant, who treated his right knee eight years ago."
Doctor Waler Pascale, who visited Ronaldo at a Milan clinic, said it was impossible to establish a time for his recovery, "but if things go well he will recover his skills of great player."
Meerssemann said the footballer had had problems in the joint.
"This knee has bothered him for a year and last week he felt something, but it did not prevent him from training (Tuesday)."
Ronaldo twice was operated for rupturing the cruciate ligament in his right knee while playing for Inter Milan, injuries that required surgery in November 1999 and April 2000. He was effectively sidelined for 17 months at the time.
Now, Ronaldo had just recovered from a minor injury suffered in November in his left calf. His season had a belated start due to a pulled thigh muscle sustained in July as he kicked the ball during a meeting with fans.
Ronaldo, the alltime World Cup top scorer with 15 goals and World Cup winner in 2994 and 2002, came to Milan in January 2007 from Real Madrid.