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.
Teams were already allowed five regular substitutes in the FA Cup, but with the rule change Fredericks did not count as one of those.
Diop - who will now be assessed to ascertain the extent of any potential damage - was himself a substitute after he came on for Angelo Ogbonna just 16 minutes into the fixture after he twisted his ankle.
The long-awaited trial for concussion substitutes was approved last week by football lawmakers IFAB, and the measure will also be in place in the FA Women's Super League and FA Women's Championship.
The arrival of concussion substitutes signals the FA and Premier League's desire to be at the 'forefront' of protecting footballers, and comes amid Sportsmail leading the way in asking football authorities to address the issue of football and brain injuries.
Meanwhile, Hammers boss David Moyes will be hoping for good news on Ogbonna and Diop in the next few days, with their next fixture at home to Sheffield United in the Premier League on Monday as they aim to continue their push for the European places.
