Lying flat on the Vicarage Road pitch, it seemed Anthony Martial's game could be prematurely over.
The Manchester United forward had clashed with Watford defender Daryl Janmaat's head as they both soared to reach the descending ball. While Janmaat made the connection for the header, Martial felt the full force of the impact with his Premier League opponent.
But after receiving attention from United's doctor, Martial was able to continue playing in Sunday's game — for a short time, at least. Seven minutes later, Martial was off the field for good.
Although it was a crunching tackle that brought Martial down again in the 34th minute, the Frenchman was still clearly disorientated from the earlier collision. After United's medical team carried out further checks on the player's vision, Martial left the pitch clutching his forehead with his left hand.
It immediately raised questions about why Martial had remained in the game, if concussion was suspected. The English Football Association's guidelines , introduced in November, are clear: "There must be no return to play on the day of any suspected concussion."
United manager Jose Mourinho was vague after the 3-1 loss at Watford, saying he did not know if Martial was concussed. Martial's only comments on the incident came through his social media accounts on Tuesday, writing alongside a thumbs-up emoticon: "Feel much better today."
The continued uncertainty about Martial's condition has prompted scrutiny of United's adherence to concussion protocols. Headway, a British brain injury association, called for an independent review into how concussion guidelines are implemented.
"Too often over the course of the past few seasons we have seen players return to games following a head injury, only to be substituted a short time later showing more obvious signs of concussion," Headway said in a statement, warning about the dangers of a player receiving a secondary blow.
But Ian Beasley, who helped to create the FA guidelines, cautioned against a rush to judgment against clubs or his fellow doctors.
"Things happen on the pitch that are easy to see from the stands and on camera but are not easy to see on the touchline," Beasley told The Associated Press.
"It's like someone sprains their ankle. You see how it goes and within two minutes they put their hand up and say they can't carry on. You haven't done any more damage it but at least you know what the situation is."
Snap assessments are required during matches, although precise tests for concussion don't exist. Players are asked standard questions by the medic, including:
- What venue at we at?
- Which half is it now?
- Who scored last in this match?
"All the questions can be answered normally and later inside they start to feel the effects, five to 10 minutes later, sometimes two hours later," said Beasley, who was England team doctor until June when he left the FA.
Doctors like Beasley can receive calls from players long after a game suspecting concussion that wasn't immediately obvious at the time.
"Concussion is not a straight forward thing," he said. "So people will ring you two hours after you have got home in the evening after the game and say, 'I've got a headache and I feel a bit groggy,' and they've got concussion. It's an evolving condition."
FIFA allows matches to be stopped for three minutes while team doctors assess players with head injuries.
The FA protocols were introduced following a high-profile incident in the 2013-14 Premier League season when Tottenham goalkeeper Hugo Lloris was allowed to return to the pitch by then-coach Andre Villas-Boas despite appearing to lose consciousness when he was struck on the head by an opponent's knee.
The FA only provides guidelines and cannot punish a club suspected of being too hasty in allowing a player to continue. There is no evidence United was at fault on Sunday.
But it's a reflection of the growing awareness of concussion that head injuries during globally-televised games, such as the Premier League, can prompt an immediate stream of comments online from fans and pundits debating medical procedures.
"The more people who focus on concussion is fantastic," Beasley said. "But I think our care of concussion is light years ahead of what it was two or three years ago. Everybody notices it more."