With English football's first extra permanent concussion substitution, we have proof of why it doesn't work.
Issa Diop hurt his head after 36 minutes in West Ham's FA Cup tie with Manchester United. It was nasty. Diop was on the ground for two minutes, and commentators said it looked as if Anthony Martial's forehead was swollen after their clash. And yet they continued.
Watch Tuesday's match again and, at the half-time whistle, Diop can be seen walking towards the tunnel more slowly than his team-mates and holding his head.
Issa Diop suffered a nasty clash of heads with Anthony Martial in the FA Cup on Tuesday night
He was treated but then played on before he was substituted for concussion at half-time
At half-time, he is substituted for concussion.