Despite his youth, Mbappe flew to Russian Federation on Monday as one of the tournament's most recognisable figures and was expected to play a key role in France's bid to win their second World Cup.
There are no details thus far as to the extent of the injury, but the 19-year-old is expected to undergo tests after his early exit.
Mbappe eased fears of a serious injury, posting on social media: "I'm fine, it was just a knock, so not serious. And let my friend Rami [know] it was not nasty".
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Kylian Mbappe was forced off injured during a France training session after suffering a knock from Adil Rami.
Rami turned the camera to Mbappe and shouted from across the treatment room: "Hey, Kylian are you OK?"
The incident left Rami red-faced, with France opening up their World Cup campaign in Russian Federation on Saturday, when they play Australia at 11am BST.
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Les Bleus kick off their World Cup campaign against Australia on June 16 in Kazan.
France is in Group C along with Denmark, Peru and Australia and is among the favorites to win the World Cup.
Mbappé has started each of France's last seven matches, scoring three times.
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