Japan midfielder Wataru Endo was stretchered off following a clash of heads during Stuttgart's 2-1 home win over Hertha Berlin on Tuesday in the German Bundesliga.
The potential layoff for Japan's regular starter would be a huge blow to manager Hajime Moriyasu with their World Cup Group E opener against Germany just two weeks away on Nov. 23 in Qatar.
Starting in defensive midfield, Endo was having a solid outing at Mercedes-Benz Arena until he was floored after going up for a ball with Ivan Sunjic in the 77th minute.
Endo received treatment before he was substituted five minutes later. Local paper Stuttgarter Nachrichten reported Endo was briefly unconscious.
Interim manager Michael Wimmer said Endo suffered a suspected severe concussion, while their sports director Sven Mislintat hoped nothing is broken and Endo can play at the World Cup.
Moriyasu announced his 26-man squad on Nov. 1 but has already been forced into making a change after an Achilles tendon injury ruled Huddersfield defender Yuta Nakayama out for the season. He has been replaced by Shonan Bellmare forward Shuto Machino.
Bochum forward Takuma Asano and Borussia Monchengladbach defender Ko Itakura, both picked in Japan's World Cup squad, remained sidelined with respective knee ligament injuries as Bochum won 2-1 at home on Tuesday.
