Ryan Fredericks has become the first ever concussion substitute to be used in English football after replacing West Ham team-mate Issa Diop.
The French centre-back clashed heads with compatriot Anthony Martial in the first half of the Hammers' fifth-round FA Cup tie against Manchester United at Old Trafford.
New rules introduced on Saturday - which saw the Premier League become the first division in the world to introduce concussion substitutes - now allow teams to make two permanent changes per game in the event of head injuries.
And with Diop unable to play any further part in West Ham's bid to reach the quarter-finals of the competition, he was replaced by full-back Fredericks at half time.