Trials for concussion substitutes are set to begin in English football next season despite authorities aiming to implement them from January.
The new rule will allow permanent substitutions to be made if a player suffers a head injury, even if all replacements have already been used.
To avoid potential abuse of the rule, opposition teams will also be able to make a change at the same time.
The trials are set to be discussed by law-makers on 23 November.
A final decision will then be made when the International Football Association Board (Ifab) - football's rule-making body - meets again in December.
Trials could begin in January 2021 "for any competition that is interested in taking part", according to world governing body Fifa and Ifab.
It is understood the Premier League and Football Association would be in favour of changes, but are awaiting further details and there are doubts about whether they would be implemented midway through a season. Any new protocol for the Premier League would be subject to a vote by clubs.
Concussion substitutes were due to take place in the football competitions at the 2020 Tokyo Olympics before the Games were postponed until 2021 because of coronavirus.
Brain injury charity Headway said a potential delay until next season was a concern, and it would prefer temporary substitutions being used, as they are in rugby union, where doctors are given 10 minutes to assess a head injury.
Headway deputy chief executive Luke Griggs told BBC Sport: "I can understand why these things need to be trialled in order to get the infrastructure around it complete.
"But there has been meeting after meeting and proposal after proposal about head injury substitutions for so many years and here we are at the end of 2020 and still nothing has happened.
"Five substitutions were brought in to help with the condensed calendar because of coronavirus, so it shows football can make rapid change when it wants to."
On the subject of temporary substitutions, he added: "It is not a perfect solution, but it gives you more time to make a considered decision about a head injury.
"Concussion is hard to diagnose and needs a degree of honesty from the player, but with a temporary substitution allowed - which of course would turn into a permanent substitution - it avoids the risk of a snap judgement.
"If it's worked in rugby, then why can't it work in football?"
Fifa's concussion expert group has said the protection of players is its "main goal and that a clear and uniform approach is needed, which can operate effectively at all levels of the game".
Fifa added: "Clear, scientific and football-based feedback will be collected to enable Ifab, Fifa and the expert groups to evaluate the impact of these trials and make appropriate recommendations."
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