The Professional Footballers' Association (PFA) has called for the introduction of temporary concussion substitutes following a head injury suffered by Leeds United's Robin Koch in Sunday's 4-2 Premier League defeat by Manchester United.

Defender Koch was left bloodied after a clash of heads with Scott McTominay early in the match and continued to play on with heavy bandaging before being substituted in the 31st minute.

The players' body said the existing concussion protocols were failing to prioritise player safety.

"The 'if in doubt, sit them out' protocol is not being applied consistently within the pressurised environment of elite competitive football," the PFA tweeted on Monday.

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"We see frequent incidents of players returning to play with a potential brain injury, only to be removed shortly afterwards once symptoms visibly worsen."

Leeds said Koch passed all of the on-field concussion screening tests that are part of the Premier League protocols.

"The medical staff at Leeds United have always been in favour of temporary substitutions for head injuries, as it would allow the staff more time to assess an injury and allow a period for symptoms to potentially develop," a club statement said.

Koch will follow the concussion protocols before returning to play.

"I wanted to support the team for longer yesterday, but unfortunately I couldn't do it," Koch tweeted.

"Thanks to our medical staff for the good care. I feel much better today and will be back soon."

In January last year, England's top flight agreed to trial permanent concussion substitutes.

According to the league's protocol, team doctors will have as much time as required to make an assessment of a player. If the player shows clear symptoms, he will be substituted and prevented from returning to the field of play.

However, the system has been criticised as it puts pressure on doctors to make snap decisions, while calls have increased for temporary substitutions to be allowed while head injuries are being assessed.

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"Introducing temporary substitutes would allow a match to restart with neither side numerically disadvantaged, reducing pressure on players and medical teams to make quick decisions on whether an injured player continues," the PFA added.

"Put simply, the current rules set by (rule-making body) IFAB are not working, and players are being put at risk."

Brain injury charity Headway also said it was "frustrated and confused" at football's response to the issue.

"Medics have a tough time when trying to make on-pitch concussion assessments. The game simply has to help them by implementing temporary concussion substitutes," Luke Griggs, Deputy Chief Executive of Headway, said in a statement.

"We need urgent answers from the Premier League as their reputation is on the line here."