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 that the concussion protocols within football 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 today.

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

In January last year, England’s topflight 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.

“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.”