Real Madrid have confirmed that midfielder Aurelien Tchouameni has been diagnosed with a stress fracture in his left foot.
Tchouameni started the game and played the full 90 minutes as Madrid ran out 2-1 winners, but posted an image of himself in a medical facility after the match to send fans into a concerned frenzy.
Madrid have now diagnosed Tchouameni with a stress fracture in his left foot, with the La Liga leaders declining to put an estimate on his return date.
Local reports, however, state that the French midfielder will be sidelined for at least six weeks and perhaps closer to two months, meaning he may not return to competitive action until 2024.
The news comes as an enormous blow to Madrid, who have not tasted defeat in any of the 12 games in which Tchouameni has started this year. In La Liga, he has started every match aside from the 3-1 defeat to Atletico Madrid.
Eduardo Camavinga now appears to be the only natural defensive midfielder in Carlo Ancelotti's squad, although Toni Kroos was used in that position at times last season.
Madrid take on Rayo Vallecano, Valencia and Cadiz in November, with matches against Granada, Real Betis, Villarreal and Alaves scheduled for December, and it remains to be seen whether Tchouameni will make it time to play in any of those fixtures.
Tchouameni also appears set to miss the remainder of Madrid's Champions League group stage campaign. Ancelotti's group leaders face Braga and Napoli in the next month before wrapping things up against Union Berlin on December 12.
Madrid are already without goalkeeper Thibaut Courtois and centre-back Eder Militao, while young midfielder Arda Guler continues to be eased back into action after undergoing knee surgery during the summer.
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This article was originally published on 90min as Real Madrid confirm huge injury blow after El Clasico victory.