Stoppage-time strikes from Declan Rice and Gabriel Jesus secured a 3-1 win for the hosts, who had equalised through Martin Odegaard just 110 seconds after Marcus Rashford had opened the scoring for United.
The late double would have been even more galling for the away side as substitute Alejandro Garnacho thought he had won it with his own strike, only to see it ruled out by a close offside VAR call.
That was just one of the close decisions ten Hag believes fell in Arsenal’s favour as the Red Devils slipped to a second defeat of the season in north London.
The Dutchman also felt Kai Havertz should have been booked for diving after seeing a penalty award overturned by VAR and that Rice’s goal came about only because Jonny Evans had been fouled – while he called for debutant Rasmus Hojlund to have been given a spot-kick of his own.
“The performance was right but the result was not on our side and definitely many decisions were against us,” he said.
“Let’s start at the penalty given but rejected. Everyone can see it’s a simulation but he did not get booked for it.
“Then the foul on Hojlund in the penalty area and I don’t think it was even noticed by the VAR. Then the disallowed goal from Garnacho.
“I think they looked from the wrong angle and it’s onside. Then the final goal. How can they allow that? It’s a clear and obvious foul on Jonny Evans otherwise he would have blocked the shot from Declan Rice. So it’s a lot.”
Despite the defeat ten Hag felt his team – without a number of injured first-team players and having also lost centre-back pairing Lisandro Martinez and Victor Lindelof to injury and illness, respectively, during the game – put in a strong showing.
“I also have a good feeling because our performance was very good,” he added.
“I won’t say it was perfect because there is definitely room for improvement. But if we see our compactness, pressing, moving with the ball and making the counters, very calm, we never gave Arsenal an opportunity to press us.
“The next stage is we could have done that better and the movement we could have done in the right moments. There were a lot of positives in this game but there is still a lot to do.”
Arsenal have now won three of their opening four games as Rice once again shone following his £105million summer move from West Ham.
He topped off a fine display with the important second goal as manager Mikel Arteta hailed the influence of the England midfielder.
“I think, a tremendous performance,” he said.
“When you look at how a holding midfielder needs to dominate his area, how he needs to break up play, how he glided the team together when they were stretched a bit.
“Then he produced a magic moment to win us the game, so, (I’m) really happy with him.
“He’s a great kid. I think he’s got a good mixture between being extremely demanding with everybody and himself, having a bit of banter and being around the staff and the boys in a really humble way. So I think he’s fitting in brilliantly.”