France forward Bafetimbi Gomis had a blackout whilst training with Les Blues on Wednesday.
The Lyon striker collapsed but regained consciousness a minute later and rejoined his team-mates after a quarter of an hour
It is the third time the 24-year-old has fainted since joining Lyon in the summer, and the second in the space of a week, and club president Jean-Michel Aulas admits it is a concern, although several medical examinations have never revealed a serious problem.
"We can't not be worried, it scares you each time," Aulas told ITele.
"A vasovagal episode (fainting) is part of the things that can happen to people who have this type of constitution.
"It's now three times that it's happened. Yes, I'm worried."
Gomis was called into the France squad as a replacement for the injured Franck Ribery for Les Blues' crucial World Cup qualifiers against Faroe Islands and Austria.