"The operation was a success and we believe he will be able to return to work in three weeks time," Real doctor Alfonso del Corral told the club's website.
Brazil doctor Jose Luiz Runco had said the player was suffering from tibial calcification around the tendon in his knee and needed surgery to remove the calcium after the World Cup finals.
Ronaldo backed Runco's diagnosis though Real said the player would not need surgery and that they had advised him to rest.
The 29-year-old striker struggled with minor health problems and suggestions he was overweight before the tournament, in which Brazil were knocked out in the quarter-finals by France.
Two goals against Japan and one against Ghana allowed him to break the World Cup scoring record with 15 goals in the finals.
