Saudi Arabia captain Sami Al-Jaber was rushed to hospital on Saturday with acute appendicitis, a spokesman for the team said.
Al-Jaber was taken to Tokyo's University Hospital where doctors immediately removed his appendix, the spokesman said.
The player will remain in hospital for two to three days.
The veteran striker, who once had a spell in England with Wolverhampton Wanderers, will now certainly miss the final group E game against Ireland.
Al-Jaber was already doubtful after missing the 1-0 defeat by Cameroon due to a knee injury he picked up in the 8-0 demolition by Germany.
To add to Saudi Arabia's misery, his replacement against Cameroon, Obeid Al-Dossari, will be out for about six months after he tore a ligament in his left knee during the 1-0 defeat.