The Brazilian playmaker, who was plagued with injuries over the past year but recovered to feature at the World Cup in South Africa, had left Real's pre-season training camp in California and flown to Belgium for checks by a specialist, the club said (www.realmadrid.com).
"He's a very popular player among his team mates and also a very good player but we want him at 100 percent so that he is happy," coach Jose Mourinho said at a news conference in San Francisco after Real beat Club America 3-2 in a friendly.
"If we have to lose him for a while it won't be a drama," the Portuguese added. "We have other players and that's that."
Kaka, 28, joined Real from AC Milan at the end of the 2008-09 season for a fee of around 65 million euros ($85.3 million). He made 25 league appearances last term, scoring eight goals.
Real play a friendly against Major League Soccer (MLS) club LA Galaxy on Saturday in Los Angeles.
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