Manchester United defender Matthijs de Ligt suffered an injury in the build-up to his side conceding a goal against Brentford.
Despite De Ligt clearly being in pain after colliding with Bryan Mbeumo, play was allowed to continue, with Kevin Schade proceeding to put Brentford in front in the 33rd minute.
Shortly afterwards De Ligt had to be substituted, with Harry Maguire replacing him.
It is the second time that such an incident has occurred involving de Ligt against Brentford this season.
In October the Dutchman was left with a bloodied head following a clash with Schade at Old Trafford.
On that occasion, Ethan Pinnock scored for Brentford while De Ligt was off the pitch receiving treatment.
Erik ten Hag, then in charge of United, received a yellow card for his furious protests at officials.
In that game United won 2-1 as they equalised through Alejandro Garnacho before Rasmus Hojlund got the winner.
However, on Sunday it was Brentford who took all three points as they won 4-3.
Mason Mount had put United in front before an own goal from Luke Shaw saw Brentford equalise.
Schade put Brentford in front in controversial circumstances and he added another goal in the second half before Yoane Wissa made it 4-1.
United threatened to stage a comeback as Garnacho and Amad Diallo scored, but Brentford held on to take all three points.
The result leaves United in 15th place in the Premier League - they have suffered 16 defeats this season.
United will conclude their league campaign with games at home to West Ham, away to Chelsea and at home to Aston Villa.
Their main focus will be on the Europa League, with Ruben Amorim's side having a 3-0 lead from their first leg of their semi-final against Athletic Bilbao.
United could be set to face Tottenham in an all-English final - Ange Postecoglou's team are leading 3-1 from the first leg against Bodo/Glimt.