Japan captain Makoto Hasebe has undergone surgery on his right knee and could be out of action for up to six weeks, according to Japanese media.
A news channel reported on Saturday that the Nuremberg midfielder had a tear in his lateral meniscus and had returned home to Japan for the operation.
Hasebe, who is vital to Japan`s hopes of making it past the group stage at the 2014 World Cup in Brazil, sustained the injury in a friendly against Steau Bucharest as his side were preparing for the Bundesliga restart.
Hasebe told reporters after arriving at Narita airport on Friday that he was unsure when he would be back in action.
"I`ve never been in this position before so I don`t know.
"I realise that the risk of getting injured is always right beside you and I am anticipating the worst. I want (the injury) to heal properly and don`t want to rush things."
Hasebe, who turned 30 on Saturday, joined Nurermberg from Bundesliga rivals Wolfsburg last September.
Japan are placed in Group C alongside Colombia, Greece and Ivory Coast at the World Cup.