"Although I had long since felt pain in my knee, I endured it and played. But my injury has got worse," Ki told reporters on his arrival in Korea.
"I received treatment for three weeks but it hasn't gotten any better. I cannot even run right now and I want to play an important role at the World Cup."
Ki, who has won 56 caps, also featured at the 2010 World Cup in South Africa.
Korea, drawn in Group H with Belgium, Russia and Algeria, will be playing at their eighth consecutive World Cup finals.
(Writing by Sudipto Ganguly; editing by Tony Jimenez)
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