It was the third time a bleeding De Ligt was ordered off the pitch and Brentford took advantage.
“It was dry blood so he was already treated for the injury,” Ten Hag said.
“I didn’t understand why he was sent off. I went over to Matthijs and he said ‘it is dry blood and I have to come off, I don’t know why.’ It was a huge moment because Brentford are very good at corners and you miss one of your best headers.”
United assistant Ruud van Nistelrooy was shown a yellow card for his protests and his argument with fourth official Gavin Ward continued into the tunnel at half-time.
Ten Hag’s team were far better in the second half and Ten Hag revealed the perceived injustice around Brentford’s goal had helped to get the best out of his players.
“Definitely, we were angry at half time,” he said.
“You see the togetherness, the fighting spirit and we add some determination to score goals. The stats show we play good football [this season] and don’t score enough but today we scored two brilliant goals.”
Victory for United will ease the pressure on Ten Hag ahead of Thursday’s trip to face Jose Mourinho’s Fenerbahce on Thursday. Languishing in 14th in the table during the international break, they are now into the top half and three points off the top six.
“Pressure is always there,” Ten Hag said. “We have to win every game and when you are 1-0 down, you have to see how we turned it around. We felt some injustice and used it as fuel. I really enjoyed the performance.”
