Patrice Evra was sent off after appearing to kick a fan in the head before his Marseille side’s Europa League game at Vitoria Guimaraes.
Footage seems to show the 36-year-old ex-Manchester United defender aiming an acrobatic kick at a fan at the side of the pitch during the warm-up.
Substitute Evra was red-carded before kick-off in Portugal, and his side started with 11 players.
Uefa’s disciplinary body is likely to rule on any further action on Friday.
French newspaper L’Equipe reported the confrontation was with Marseille supporters who had been jeering Evra for about half an hour while the players prepared for the game.
Its reporter said Evra had initially gone over to the fans to talk to them, but the situation had escalated quickly.