In the 62nd minute of his team's match against England, Uruguayan defender Álvaro Pereira took an accidental knee to the head from Raheem Sterling.
Pereira remained motionless on the ground for about a minute after the collision.
When he eventually got to his feet, doctors attempted to call for a substitution, but Pereira overruled them.
Many on Twitter immediately criticized doctors for allowing Pereira to continue playing after he appeared to suffer a concussion.
Hate seeing doctors overruled with Pereira coming back on. World soccer clueless on concussions.
Nobody can begrudge a player for wanting to stay on, but right now Pereira's brain is vulnerable. Here's hoping he avoids any bad clashes.
Allowing Pereira back on the field looks medically wrong to me. I don't care how much he protests. Get him off. People die of clots.
In Spanish, some started using the hashtag #loshuevosdepereira — "the balls of Pereira" — to praise the Uruguayan player for his toughness.
"The guy got up and asked not to be substituted out."
El tipo se levantó y pidió que no lo cambien. #loshuevosdepereira http://t.co/j0ElrTK5GZ
"The guy PASSED OUT and preferred GOING BACK TO THE GAME. Hospitals or World Cup? Álvaro Pereira can answer."
O cara DESMAIOU e preferiu VOLTAR PRO JOGO. Hospitais ou Copa? Alvaro Pereira responde #URU
"After getting hit, the doctor asked for Pereira to be substituted out. The guy asked if he was nuts and that he was not leaving the game."
Tras el golpe el medico pidio el cambio para Pereira, y se vio que le dijo que si era loco que no que no iba a salir.