Trials using permanent concussion substitutes in English football have sometimes "fallen short of their objective" and "jeopardised player health", putting unfair pressure on medical staff, the global players' union FIFPRO and England's Professional Footballers' Association have said.
In a joint letter sent to the International Football Association Board (IFAB), which determines the laws of the game, FIFPRO and the PFA said existing trials using permanent concussion substitutes should be extended to test parallel temporary concussion replacements "as soon as possible".
The Premier League began trialling the use of concussion substitutes in February in response to growing concerns about the damage caused by head injuries in football. The trials are also being used in the FA Cup and Women's Super League.
That was after the IFAB approved the use of concussion substitutions for any league wishing to take part in December, saying the trial period would run until August 2022.
Teams are allowed to make two permanent concussion substitutes even after making all their permitted changes, but FIFPRO and the PFA believe the ongoing trials should be extended to include temporary concussion substitutes from June onwards, along with the existing permanent option.
The letter highlights the cases of West Ham United's Issa Diop and Sheffield United's George Baldock, who were both assessed for head injuries but continued to play before being removed when it emerged their injuries were worse than first thought.
"These cases underline our concern that permanent substitutions do not give medical teams the appropriate environment to assess a player with a potentially serious head injury," the letter said.
"The reality of the in-game situation is loaded with pressure. Medical teams can be presented with a situation where a globally broadcast match is on hold, awaiting their assessment. They have to make a potentially game-altering decision in a multi-billion-pound industry."
The letter references a FIFPRO poll of 96 doctors at top-flight clubs in England, France and Belgium and says 83 percent of those asked favoured allowing temporary concussion substitutes to give medical teams additional assessment time without forcing teams to restart games a player short.
"Player safety and welfare is paramount, and should be the only priority. Therefore, we are writing to ask that IFAB's Board of Directors extend the scope of the 18-month trial to include parallel trials for temporary concussion substitutions starting no later than June 1, 2021."