Man United were blown away during the opening 20 minutes at the Emirates as Mesut Ozil’s goal and a brace from Alexis Sanchez gave Arsenal a dream start.
It was a shambolic display from United, who were abject from start to finish.
Van Gaal criticised his side following the clash and admitted the loss made him “emotional”.
“When you start like that you can never win a game, we lost every duel, every second ball, not tight on their midfielder, and they can play they are a fantastic team when you let them give them that space it is difficult,” he said.
“That was the worse United have been under me, we were not able to give a normal performance.
“I have never seen that of my team, that is the question I put to my players. You can not solve that now, a lot of emotion and I am emotional as well.
“I could not change anything, if it was just one player then I can take measure but it was more than half of my team who were not doing what we had agreed in our game plan.”
United had been in good form going into the clash but all their good work was undone following the dismal performance against Arsenal. Unfortunately for van Gaal, he won’t be able to get his players straight back into the training ground due to the upcoming international break.