Rudi Garcia's side face Iran on Sunday looking to build on an opening 1-1 draw with Egypt, but will do so without Manchester City player Doku.
"Due to illness, Jeremy Doku will not be part of the squad for our upcoming game against Iran," Belgium posted on social media.
Doku started against Egypt but struggled to make an impact before being replaced by Matias Fernandez-Pardo in the 86th minute.
The 24-year-old could also miss a knock-out stage match, as his wife is due to give birth to their first child in the second week of July.
Doku is one of the best young players in the Belgium squad, who are attempting to rebuild after their 'golden generation' unravelled with a group-stage exit from the 2022 World Cup.
World Cup 2026
The 2026 World Cup will be held from June 11th to July 19th in the United States, Canada and Mexico. The tournament will feature 48 national teams and will be played in 16 modern stadiums.
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