The world players' union on Friday accused World Cup organizer FIFA of failing to protect Uruguay midfielder Alvaro Pereira after he played on following a hard blow to the head that he said felt "like the lights went out."

Pereira lay motionless after colliding with Raheem Sterling's knee in the 61st minute of Uruguay's 2-1 victory on Thursday. Even though he appeared like a punch-drunk boxer, Pereira ultimately was allowed to return in the 63rd minute.

The union urged FIFA "to conduct a thorough investigation into its own competition concussion protocol which failed to protect Uruguayan footballer Alvaro Pereira."

It also suggested possible rule changes so players suspected of being concussed can be temporarily substituted while they're diagnosed.

"Football is awash with incidents in which players suffer potentially concussive blows to the head and stay on the pitch. In Pereira's case, he demanded to play on, overruling advice from Uruguay's team physician for him to be immediately substituted," FIFPro said in a statement.

"FIFPro understands that in certain moments, faced by the pressures of such an important international stage, many players would react in this way. There are times, however, when the players also require greater protection against the prospect of making any rash decisions."

"After the hit, I only recall that I was unconscious for an instant," Pereira said. "It was like the lights went out a little bit."

Teammates surrounding the motionless player signalled for a stretcher. Team medical staff came onto the field. Pereira wobbled to the sideline. Uruguay physician Dr. Alberto Pan signalled for a substitution.

Pereira seemed to be having trouble maintaining his balance. He argued and angrily wagged a finger to signal he didn't want to be substituted.

"I said 'sorry' a thousand times to the doctor because I was dizzy. It was that moment your adrenaline flowing in your body, maybe without thinking ... what I really wanted to do was to help get the result," the 28-year-old Sao Paulo FC player recalled. 

Independent doctors could help

Pereira said a team doctor and a FIFA physician checked him after the match. There was no immediate confirmation on whether he sustained a concussion, a subject that is slowly moving up the agenda in the sport.

FIFPro said it would monitor Pereira's health on Friday.

The union said it also is considering appointing its own independent doctors "for all future FIFA competitions."

In the English Premier League, Tottenham came under scrutiny in November for an incident involving goalkeeper Hugo Lloris. Currently with the France squad at the World Cup, Lloris was allowed to play on after he was briefly knocked unconscious during a game after colliding with Everton's Romelu Lukaku, despite medical advice he leave the match.