Real Madrid's Federico Valverde was taken to hospital with a head injury after an alleged dressing room incident with team-mate Aurelien Tchouameni, just three days before they face a crunch title match against rivals Barcelona.
Sources have told BBC Sport that Uruguayan midfielder Valverde was knocked unconscious in an altercation, while ESPN has reported Valverde needed stitches.
Tensions between the pair reportedly started during Wednesday's training session. Although their argument is said to have continued into the dressing room, it did not go further at that point.
Thursday's alleged altercation is widely reported to have taken place after training at the club's Valdebebas base, when France international Tchouameni is said to have confronted Valverde and a verbal argument turned physical.
An emergency meeting was called for Thursday evening with Real president Florentino Perez and his staff, plus head coach Alvaro Arbeloa and club captain Dani Carvajal.
A Real spokesperson has told Reuters the club are considering how to respond to the incident.
BBC Sport has contacted the club and the players' agents for comment.
Real face Barcelona in El Clasico at the Nou Camp on Sunday. Anything other than a win for Real would hand their rivals a consecutive La Liga title, with three games to spare.
Spanish media say the incident has been described internally by several members of staff as "the most serious ever experienced in Valdebebas".
Reports suggest the club have opened an investigation to learn what happened and that disciplinary action could be taken against both players.
A divided dressing room?
Reports of tension between Valverde and Tchouameni follow claims of other disagreements in the Real Madrid dressing room.
Earlier in the week, Spaniard Alvaro Carreras responded to numerous reports that he had an altercation with Germany's Antonio Rudiger.
Left-back Carreras did not specify which player he was referring to when he posted a statement on Instagram that read: "In recent days, certain insinuations and comments about me have emerged that do not correspond to reality.
"My commitment to this club and to the coaches I have had has been complete from day one, and it will continue to be so. Since I returned [after spells at Manchester United and Benfica], I have always worked with the utmost professionalism, respect and dedication. I have fought very hard to fulfil my dream of returning home.
"Regarding the incident with a colleague, it is a specific matter of no relevance that has already been settled. My relationship with the whole team is very good."
Supporters have also showed their discontent with star striker Kylian Mbappe this week, after he spent some recovery time in Sardinia with his girlfriend.
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