Belgium captain Vincent Kompany missed training on Thursday because of a groin strain, three days ahead of his team's Group H game against Russia at the World Cup.
Kompany stands at the heart of the Belgium defense and missing the Manchester City stalwart would be a big handicap for the group leader.
Coach Marc Wilmots called it a "slight" strain and said Kompany was working on an individual program with the medical staff.
Playmaker Eden Hazard made a return to training and showed no ill effects from a toe contusion sustained in Tuesday 2-1 victory over Algeria.
Belgium can advance to the knockout rounds if it beats Russia on Sunday.