Alvaro Pereira apologised to Uruguay's team doctor after playing on against England despite being knocked out by Raheem Sterling.
Pereira was left unconscious on the pitch around the hour mark on Thursday - after being struck in the head by Sterling's knee - but when he awoke, the full-back angrily pushed away medical staff, who wanted to substitute him.
The full-back attempted a sliding tackle on Sterling and got hit on the side of the head by the Liverpool winger's knee.
Despite looking visibly shaken, Pereira completed the Group D fixture at Sao Paulo's Arena Corinthians, which raised plenty of questions about FIFA's concussion policy.
While Pereira conceded after the game that he may not have done the right thing, the 28-year-old remained most interested in Uruguay's 2-1 win rather than his own health.
"It was a moment of madness... I've apologized to the doctor because I know it's his job to look after the players," Pereira said.
"I went back on dizzy but, in the heat of the moment, with a hot head, you don't think properly.
"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."
Pereira's decision to play on was condemned on Twitter with FIFPro's director of communications Andrew Orsatti slamming FIFA's lack of regulations regarding concussion.
The danger of concussion has become a major issue in American football, rugby union and Australian football over the past five years with more stringent protocols being introduced to protect players.
"That Pereira was allowed to play on without a thorough sideline concussion assessment is just appalling. Lives are at stake, FIFA. Wake up," Orsatti tweeted.
Soon after returning to the pitch, Pereira crunched England's Glen Johnson from behind but somehow escaped sanction from the referee.
Dr Aaron Gray, the sports medicine physician at the University of Missouri, tweeted that Pereira's tackle was a clear symptom of concussion.
"No one in right mind makes that challenge on ball rolling out of bounds. Irritability, anger, confusion (are) all concussion symptoms," Gray wrote.
"Pereira's rash tackle after returning likely caused by effects of concussion and could have caused him to be sent off."comments