Kylian Mbappe was forced to leave France's training ground on Tuesday after a clash with Adil Rami, but he later took to social media to calm injury fears.
The Paris Saint-Germain forward crumpled to the floor after the challenge from Rami and remained on the ground for several minutes as the medical staff assessed the injury.
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.
But Mbappe took to Twitter after the incident, saying: "I'm fine, it's just a knock so it doesn't matter but thank you for your messages.
"And leave my friend [Adil Rami] alone, it wasn't nasty."
In another sign of optimism, Rami said his team-mate was fine when he asked Mbappe how he was in a video posted on the defender's Instagram page.
Rami turned the camera to Mbappe and shouted from across the treatment room: "Hey, Kylian are you OK? Yes, yes!"
The incident comes just four days before Didier Deschamps' men begin their campaign in Russia with Saturday's clash against Australia. They will then face Peru and Denmark to complete the group stage.
If Mbappe is unfit to compete in Russia, Deschamps will have until 24 hours before Les Bleus' opening fixture to replace him in his 23-man squad and could call on Alexandre Lacazette or Anthony Martial from his reserve list to step in.
MONDIAL-2018 : L’attaquant français Kylian #Mbappé quitte prématurément l'entraînement après un choc au niveau de la cheville avec Adil Rami. (ESPN) pic.twitter.com/bsE7hwDmZz — Infos Françaises (@InfosFrancaises) June 12, 2018
Mbappe has been a pivotal player for France over the last year and has started each of the last seven friendlies, getting three goals and four assists in that time.
The 19-year-old star enjoyed an excellent debut season at PSG, scoring 21 times and setting up 16 in 46 games in all competitions.