The Nagoya striker underwent tests at a hospital after coming off at half time during their 2-1 away defeat to Urawa Reds on Wednesday, the Kyodo news agency said.
The 30-year-old, who scored a goal in Japan's 4-1 loss to Brazil at the 2006 World Cup, is expected to be named in the "Blue Samurai's" 23-man squad for the June 11-July 11 finals in South Africa.
Japan have been drawn in Group E with Netherlands, Cameroon and Denmark.
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