ATLETICO MADRID defender Robin Le Normand suffered a "traumatic brain injury" during their derby draw with Real Madrid.
The Spain international, 27, sustained the blow in the latter stages of Sunday's clash at the Wanda Metropolitano.
Atletico have confirmed Le Normand has suffered a subdural hematoma - which is when blood collects between the skull and the surface of the brain.
It is understood he has not travelled to Portugal for their Champions League clash with Benfica on Wednesday.
A from the club read: "Robin Le Normand suffered a severe blow to the head in the final minutes of the Madrid derby, for which he underwent a series of tests and was assessed by specialists who determined that he suffered a traumatic brain injury (TBI) with subdural haematoma.
"In the process of returning our player to competition, the guidelines set out in the protocol established for this type of trauma will be followed."
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are reporting a "serious injury has been ruled out", but his immediate absence will be a disappointment for Atletico manager Diego Simeone.
Le Normand has started all but one of Atletico's nine matches so far this season.
He also featured for Spain against Serbia and Switzerland in their two Nations League games last month.
Atletico's 1-1 draw with Real Madrid ended in chaos after the match was suspended for around 15 minutes due to home fans throwing objects onto the pitch.
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Thibaut Courtois was targeted with missiles as he attempted to take a goal kick.
The referee sent both sets of players down the tunnel, while Atletico's players and head coach Simeone urged the fans to stop.
The match eventually resumed with Real Madrid leading 1-0, but Atletico equalised in stoppage time through Angel Correa.