Uefa has contacted the French Football Federation to find out why France's Benjamin Pavard carried on playing against Germany at Euro 2020 despite losing consciousness briefly.
The Bayern Munich defender, 25, collided with Germany's Robin Gosens in the second half of France's 1-0 win.
Pavard said: "I took a hell of a shock. I was a little knocked out for 10 to 15 seconds. After that it was better."
Uefa wants to know what happened and whether protocols were followed.
Prior to Euro 2020, all teams, including doctors, agreed to a concussion charter stating: "We confirm that if a player of our team is suspected of having suffered a concussion, he will be immediately removed from the pitch, whether in training or match play."
Global players' union Fifpro said it was in touch with Uefa "to find out why the concussion charter was not applied".
Brain charity Headway says the handling of the incident was "sickening to watch" and called on Uefa to "immediately explain how it was allowed to happen and what action it will take to ensure something similar does not occur in the future".
Fifpro has called for football to use temporary concussion substitutes so that players can be replaced while they are being assessed for head injuries.
Trials for concussion substitutes, which do not allow a player to be replaced while they are being assessed but offer additional permanent replacements, have taken place this season in England and other countries.
But Uefa is not carrying out a trial during the European Championship, having used it in a recent under-21 tournament.
Questions about player welfare have increased after Christian Eriksen suffered a cardiac arrest during Denmark's opening game against Finland on Saturday.
Headway chief executive Peter McCabe added: "This is another example of football authorities failing to protect the short and long-term health of a player.
"It was plain for all to see that Pavard was unable to protect himself from the fall. Pavard's later statement that he lost consciousness confirms the seriousness of the incident.
"We have continuously been told that football's concussion protocols are fit for purpose and that temporary concussion substitutes are not necessary.
"But here we have yet another example where it is simply not credible to suggest that a concussion could not be 'suspected' or a possible consequence of the impact."
Uefa to review homophobic banner in Hungary-Portugal match
Meanwhile, Uefa has also received a report on a homophobic banner at Tuesday's Euro 2020 match between Hungary and Portugal.
Images of the flag circulated on social media, before being picked up by anti-discrimination group Fare.
Fare also criticised Hungary for the booing of the Republic of Ireland team for taking the knee - as an anti-racism gesture - during a pre-tournament friendly in Budapest on 8 June.
Hungary's parliament passed legislation on Tuesday that bans the dissemination of content in schools deemed to promote homosexuality and gender change, amid strong criticism from human rights groups and opposition parties.
Neither Hungary's football association (MLSZ) nor Uefa responded to a request for comment.