The defender later admitted it was "a moment of madness".
Pereira was clattered in the face by England forward Raheem Sterling's knee an hour into the game and he lay unconscious as medical staff rubbed his chest and tried to revive him.
When he got up, staggering along the touchline, Pereira furiously pushed away medical staff who wanted him substituted and he forced his way back on to the pitch.
While his coaches let him play out the final 30 minutes of the game and he drew praise from other Uruguayan players, Pereira himself conceded it was the wrong thing to do.
"It was a moment of madness... I've apologised to the doctor because I know it's his job to look after the players," Pereira said after Uruguay won 2-1 with a Luis Suarez double.
"I went back on dizzy. But in the heat of the moment with a hot head you don't think properly," he said.
"Still, it was a time to help the team and to get a result. And the most important thing is that we got the result."
The incident prompted outrage on social media with the FIFA doctor at the game not stepping in to force a substitution despite the obvious concussion.
@concussionblog Decisions like Pereira set back so much progress with concussion awareness at youth level. Just 'man up' and play on = wrong
— Andrew Fleming (@AndyFleming1980) June 19, 2014
No one being motioned off the bench for Uruguay, so it looks like Pereira is going to keep playing, and that is simply negligent.
— Mike Foss (@themikefoss) June 19, 2014
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. — Teju Cole (@tejucole) June 19, 2014
The incident raised questions about whether Uruguay's bench took his head injury seriously enough.
Head injuries in sports have come under much closer scrutiny in recent years and campaigners quickly condemned the decision to let Pereira play on.
"That incident today is the kind of thing that happened a long time ago, its borderline barbaric," said Taylor Twellman, a former United States player who suffered five major concussions in his career and now campaigns for more controls in the sport.
"When a player is concussed he doesn't know what has happened - very dangerous to rely on what a player wants," Twellman told Reuters.
"Second impact syndrome is the most dangerous thing. If he goes down again he could die - that is science. You're playing with fire, you are playing with someone's life."
Uruguay goalkeeper Fernando Muslera said he was worried when he first went over to help Pereira.
"When I saw his eyes, I got scared and Diego (Godin) and I both began to call for the medics," he said. "He's an important player who loves the shirt and if he can keep playing he will. He is never going to abandon the pitch."
- Sports & Recreation
- Alvaro Pereira