The ankle injury suffered by Real Madrid striker Cristiano Ronaldo continues to be a cause for concern amid rumors that he will be out of action for at least another month.

The most expensive player in the World damaged his right ankle playing a World Cup qualifying match against Hungary last month.

It had at first been hoped that he would be fit to have played in Real Madrid's Champions League game in Milan on Tuesday. However, tests last week ruled that possibility out.

Now further tests carried out this week by his club also rule out any chance of Ronaldo playing in the city derby against Atletico Madrid this Saturday night.

It appears that his injury is linked to the lateral internal ligament, close to the scar he bears following an operation he underwent while still at Manchester United in the summer of 2008.

Sports paper Marca said on Thursday that Ronaldo will have to be out of action for at least another month. That would keep him out Portugal' s World cup play offs against Bosnia on Nov. 14 and 18 and also out of the first Barcelona - Real Madrid match on Nov.29.

However, the paper adds that should his current injury reactivate the problems Ronaldo suffered a year ago, it is possible he will need another operation and that could keep him sidelined until into the New Year.