The 33-year-old Gangwon FC forward was stretchered off the field during Saturday’s match against Gyeongnam and team doctors had hoped he would quickly recover, according to AFP.
But later testing revealed more severe damage that will take him out of the game for weeks with the World Cup kicking off on June 14.
Lee’s inability to play in Russia next month will pile pressure on the Taeguk Warriors who have already been battered by injuries.
The squad have also lost first choice centre back Kim Min-ja, key playmaker Yeom Ki-hun and Dijon FCO midfielder Kwon Chang-hoon.
“Lee Keun-ho had been preparing for this World Cup thinking it’d be his last,” South Korea coach Shin Tae-yong told reporters according to Yonhap.
“It’s quite disappointing. I was hoping he would be a great leader as the team’s oldest player.” South Korea have a devilishly tricky World Cup group.
The Koreans crashed out in the first round of the 2014 tournament in Brazil and have their work cut out to avoid the same fate in Russia, where they have been drawn alongside holders Germany, Sweden and Mexico.
They were already heavily relying on their Tottenham talisman Son Heung-min to bring his sparkling English Premier League form.
Lee would likely have been the first choice to play alongside Son up front. Son almost single-handedly led his team to the Asian Cup final in 2015, where they lost to hosts Australia.
South Korea made history with their astonishing run to the 2002 World Cup semi-finals but the current crop of players have struggled to hit those heights — or come anywhere close.