Matthjis de Ligt suffered a nasty head injury early in the game when he collided with the knee of Brentford forward Kevin Schade, leading to lengthy medical attention to stop the bleeding.
He was forced to go off for treatment on two other occasions, the second of which in first half stoppage time saw Brentford score from a corner as De Ligt fumed on the sideline.
Ten Hag and his assistant Ruud van Nistelrooy were booked for their arguments with fourth official Gavin Ward and the United boss believed it was a turning point that spurred United on to their first Premier League victory in more than a month.
'We were mad at that, we were angry at half-time,' Ten Hag explained.
Erik ten Hag claimed Manchester United 's 'mad and angry' players used the injustice around Brentford 's goal to inspire a much-needed victory
Matthjis de Ligt suffered a nasty head injury early in the game when he collided with the knee of Brentford forward Kevin Schade
Ten Hag and his assistant Ruud van Nistelrooy were booked for their arguments with fourth official Gavin Ward
'It was not perfect but we controlled the game. We didn't concede chances and felt it was an injustice to concede as we did.
'Every game we win is important and I think again this team you see the togetherness and the fighting spirit.
Asked specifically why he and his coaching staff were so irked by the insistence that a bleeding De Ligt, who kicked a water bottle in frustration when Ethan Pinnock scored, was forced off three times for treatment, Ten Hag doubled down on his reasons.
'It was dry blood,' he said. 'So it was already treated and just dry blood so none of us understood why he was sent off. I went over and asked him.
'It was a huge moment as Brentford is very good at corners and when you miss one of your best headers of course you are disappointed.'
United rise to 10th with this victory, with second half goals from Alejandro Garnacho and Rasmus Hojlund the difference, and Ten Hag is acutely aware of the need to build on this, rather than allow it to become another false dawn.
'There is always [room for improvement]. It is never good enough [to win one game] but the two goals we scored were high quality and we are seeing this team has the capacity to score very good goals.
United rise to 10th with this victory, with second half goals from Alejandro Garnacho and Rasmus Hojlund (pictured, right) the difference
'It was dry blood,' ten Hag said. 'So it was already treated and just dry blood so none of us understood why he was sent off'
'When you score and you win you add some belief, some confidence and that is what we needed.
'The start of the season we had some good performances. You see the clean sheets we had and we created enough chances but you don't score and win games and everyone is negative and disappointed.