The issue of the concussion protocol reared its ugly head again on Saturday afternoon, as Anthony Martial re-entered the action in United's clash with Watford following what looked like a head injury.
Martial, who remained on the field for roughly four minutes following his injury, was eventually substituted for Ashley Young, but had returned to the field despite looking far from his best.
Following United's 3-1 loss, Jose Mourinho was quizzed on whether Martial was concussed, responding "I don't know. I really don't know. You are asking about things I cannot tell you.
"I didn't know why he came off, whether he was injured in his knee, his ankle or his head. I don't know. I just know, in that moment, he is not in a condition to carry on. I don't know why. I just know I had to change. I don't know yet what happened."
Premier League concussion guidelines state that "anyone exhibiting symptoms must immediately be removed from playing" and that "there must be no return to play on the day of any suspected concussion."
Martial received attention to his head following the exchange with Watford's Daryl Janmaat, which resulted in the Frenchman slamming his head into the turf, but the club's doctors seemed to believe he was fit to continue, a decision which would appear to have been proven wrong just four minutes later.
United, who equalised through Marcus Rashford following Ettiene Capoue's fourth goal in five games, collapsed towards the end of the game, with Watford substitute Camilo Zuniga making a telling impact.
The Colombian netted the 83rd minute winner, having been on the field for just one minute. He was later brought down inside the area by Maroune Fellaini, with Troy Deeney finishing the spot kick in the 95th minute.
The loss spelled a third successive defeat for Mourinho, only the third time he's put together such a run since 2002.
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