LONDON -- Lying flat on the Vicarage Road pitch, it seemed Anthony Martials game could be prematurely over.The Manchester United forward had clashed with Watford defender Daryl Janmaats 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 Uniteds doctor, Martial was able to continue playing in Sundays 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 Uniteds medical team carried out further checks on the players 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 Associations 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. Martials 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 Martials condition has prompted scrutiny of Uniteds 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.Its like someone sprains their ankle. You see how it goes and within two minutes they put their hand up and say they cant carry on. You havent 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 dont 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 wasnt 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, `Ive got a headache and I feel a bit groggy, and theyve got concussion. Its 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 opponents 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 its 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. 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