The new concussion substitute rule has 'failed its very first test', according to brain injury charity Headway, who say the guidelines are 'deeply flawed'.
Ryan Fredericks became the first concussion substitute in English football on Tuesday night when he replaced Issa Diop at half-time of West Ham's FA Cup defeat at Manchester United.
Diop clashed heads with Anthony Martial in the 36th minute but, after receiving on-pitch treatment for two minutes, carried on playing until the break, before he was eventually withdrawn under the new rule.
Permanent concussion substitutes are currently being trialled in English football and allow teams to make two permanent subs in the event of head injuries, in addition to their three regular changes.