"It hurts when I move my leg and it's pretty tough," Nakamura said after arriving at Narita airport, referring to a sprained right ankle he sustained in the Scottish Cup final against Dunfermline on Saturday.
Celtic defeated Dunfermline 1-0 in Saturday's Scottish Cup final to complete a league and cup double.
Nakamura earned both the Scottish Player of the Year and Scottish Football Writers' Player of the Year awards for his outstanding performance throughout the season.
Japan faces Montenegro in Shizuoka on Friday and then take on Colombia on June 5 in Saitama in its final warm-up for the defense of the Asian Cup in July.
Ivica Osim's team has been drawn against 2006 Asian Games gold medalists Qatar, the United Arab Emirates and Vietnam.
Flirtation with Turkey turned out to be disastrous for Russia, but as long as Russia is in the game, the stakes should be high