When Arsenal's Rob Holding went off during his side's 1-0 loss to Manchester City at the Emirates Stadium, he made history as the first player in the Premier League to be replaced via a concussion substitution.
There were around 10 minutes of the encounter remaining when he was caught on the head by Joao Cancelo's studs.
After a brief period of treatment, Arsenal brought David Luiz into the fray, making the Brazilian the first player to be introduced via the law in the league.
It proved to be a wise decision as Holding was confirmed to have concussion the following day and was forced to miss some training as a result.
"That's what they told me," manager Mikel Arteta confirmed. "He will miss a few days I think."
Previously, West Ham had taken advantage of the law in an FA Cup tie to substitute Issa Diop.
What are concussion substitutions?
Concussion substitutes have been introduced in a bid to stop clubs taking undue risks with players who have head injuries. There are concerns that footballers have played when concussed, and there are fears for their long-term health as a result.
With concussion substitutes available, clubs are given another two potential changes to use over the course of a match if a player is suffering from suspected concussion.
It is up to the discretion of club doctors to decide whether a player falls under this category.
What is concussion?
According to the NHS: “Concussion is a temporary injury to the brain caused by a bump, blow or jolt to the head.
“It usually only lasts up to a few days or weeks, although it sometimes needs emergency treatment and some people can have longer-lasting problems.
“It's important to avoid head injuries as repeated concussions or blows to the head have been linked to serious problems, including a brain condition called chronic traumatic encephalopathy.”
What happens when a concussion substitute is made?
The process for a concussion substitute to be made is slightly different to the normal substitution procedure. Instead of the usual white form handed to the fourth official, clubs will use a green form, indicating the difference between the types of change.
These players are permanently removed from the game.
Additionally, it should be noted that if a concussion substitute is made, the opposition are given one additional change.
Do clubs have to make a concussion substitute when a player has a head knock?
Although clubs are compelled to take players suffering from a head injury off, they do not necessarily have to use one of their concussion substitutes to do this.
Why would they not? Simply because it would offer their opponents a chance to use that additional substitute.
Let’s use the Aston Villa v Arsenal example again. If a Villa player suffered a suspected concussion after 85 minutes, they would be granted the opportunity to change them. Of course, if they had used their full complement of substitutes, they would have to dip into their concussion change pool, but if they had only used two of their regular changes, the player could be brought off in that manner.
By using a regular substitution, Villa would be depriving their opponents of having the luxury of making an additional change.
Who was the first to use a concussion substitute?
Towards the end of the first half, Issa Diop and Anthony Martial were involved in a nasty clash of heads during a corner-kick which left both players on the floor needing medical attention.
Martial was able to continue, but Diop showed signs of concussion and was subsequently replaced at half-time by Ryan Fredericks.
Diop himself was already a substitute, having entered the action early in the first half when Angelo Ogbonna suffered an injury.
The West Ham Twitter feed confirmed that the Diop substitution was due to concussion, which meant that it did not count in their usual substitute allocation.
However, Diop was allowed to play on for seven minutes of action before being taken off at half-time, which, according to some, presents the first failure of the new ruling.
Peter McCabe, Headway CEO, on Diop injury: "Shows how deeply flawed new protocol is. Had FA taken advice of Headway + others by introducing temp subs, Diop wouldn't been at risk of exacerbating injury to brain during 7 mins he was allowed to play on. New protocol failed 1st test"— Simon Stone (@sistoney67) February 10, 2021
How long does the concussion substitute trial last?
The Premier League will trial these changes until at least the end of the 2020/21 season. It can be extended until the end of the 2021/22 campaign, however.
Why are concussion substitutes only in the Premier League and FA Cup?
It is important to stress that this is only a trial and not yet a permanent law of the game. The Premier League is the only major league taking part in this trial, although less celebrated competitions are testing it. It is also being tested in the FA Cup.
One of the reasons that Premier League clubs are doubtless so keen to implement it is because they are one of the few major competitions in which there are only three substitutes permitted this season.