Leading brain injury charity Headway has reiterated its belief that football's new concussion rules are "deeply flawed" following their domestic application for the first time on Tuesday night.
Under the rules which allow clubs to make up to two additional, permanent substitutions in the event of head injuries, West Ham's Issa Diop was replaced following a clash of heads with Manchester United's Anthony Martial at Old Trafford.
But Headway chief executive Peter McCabe says the new rule - adopted by the Football Association for the remainder of this season's FA Cup fixtures, as well as by the Premier League - does not go far enough.
McCabe highlighted his organisation's concern that Diop had initially been cleared to continue playing following an on-pitch consultation, before being substituted shortly afterwards at half-time.
McCabe said: "The decision to allow Issa Diop to return to the field of play after being assessed for concussion in just two minutes, while still on the pitch, shows just how deeply flawed this new protocol is.
"When this rule was introduced to much fanfare, we warned that it would make very little difference in terms of protecting players from the risk of more serious injury."
Headway has consistently maintained that the rule would have been much more effective if it enabled temporary substitutes - thus theoretically allowing Diop to leave the field immediately in the knowledge he could return if deemed safe to do so.
"Had the FA followed the advice of Headway and other leading experts by introducing temporary substitutes, as successfully used in other sports, Diop would not have been at risk of exacerbating the injury to his brain during the seven minutes he was allowed to play on before half-time," added McCabe.
"Instead, he could have had a longer assessment, which would have included the 15-minutes of half-time, in a quiet treatment room. This would have given the medical team more time to make their decision.
"The new protocol has failed its very first test. The FA must act now to alter the rule and introduce temporary concussion substitutes to avoid such unacceptable risk being taken in the future."
The FA has been contacted for comment.