South Korea midfielder Han Kook-Young was ruled out of the Olympics on Tuesday after breaking his left foot in training.
The 22-year-old was withdrawn from Korea's 18-man squad after a scan on Tuesday.
South Korea, who face Mexico at St James' Park on Thursday, want 22-year-old midfielder Woo Young-Jung to replace Han, but are waiting to receive approval from FIFA.
"The player has fractured his left foot and will not take part in the tournament," said coach Hong Myung-Bo.
"It is very unfortunate because it is a short time to prepare a new player ahead of such a big tournament. But we are well motivated and united so it's not a big obstacle."
"We are fully aware of how important and significant the first match against Mexico is, so I hope we will get ready for the match as normal."