The Real Madrid forward was forced out of his side's training camp in the United States and is now in Antwerp where he may require a knee operation.
Take a break: Kaka sits on the ball during training with Mourinho in Los Angeles
Kaka complained of discomfort in his knee during a routine medicalcheck-up after linking up with the Spanish club following hisparticipation in the World Cup.
Further tests revealed the new injury.
Walking wounded: Kaka after his operation
'It is suspected that there may be an injury to the meniscus, which could require arthroscopic surgery,' Madrid said on their website.
'Kaka has travelled to Belgium accompanied by club doctor Carlos Diez and will be examined by a specialist in Antwerp.'
Blues boss Carlo Ancelotti signed Kaka for AC Milan in 2003 and has admitted he would love to have him in his team again.
But former Chelsea manager Jose Mourinho, now in charge at Real Madrid, would be reluctant to let the 28-year-old leave the Bernabeu. And Mourinho refused to be too downbeat about Kaka's injury.
The Portuguese said: 'If we have to lose him for a while, it won't be a drama - we have other players. But we want him to be at 100 per cent so he is happy.
'He is a player who is much loved by his team-mates - he is one of the best players in the world.'
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