Erik ten Hag and Co were left furious when Ethan Pinnock scored Brentford's opener, moments after Matthijs de Ligt came off to receive treatment on his head, meaning he couldn't defend the corner.
Manchester United conceded their ninth goal in eight Premier League games, with Pinnock heading in from a corner on the brink of half-time.
In reaction, Ten Hag and Ruud van Nistelrooy were spotted having a word with the fourth official Gavin Ward. It was the third time that the Dutch defender came off the pitch as his head was bleeding and that has been the protocol for years.
Lacking presence in the box, Pinnock found space and strolled forward to head the ball past Andre Onana. READ MORE: Man United get Mohammed Kudus boost as West Ham suspension timeline laid out READ MORE: I spoke to Man United player – he warned of manager who dressing room would eat alive It was clear to see how much De Ligt's absence was felt during the goal.