Slaven Bilic believes Phil Jones' tackle on Sofiane Feghouli was more dangerous than the challenge which saw the West Ham midfielder sent off.
Feghouli saw red after 15 minutes at the London Stadium when he clashed in a sliding challenge with Jones.
And while Sky Sports pundits slammed the sending off, Manchester United went on to win 2-0 and earn their sixth league win in a row.
West Ham boss Bilic believes Jones "made a meal" of the challenge, with his reaction helping Mike Dean make his decision.
"It was never a red card to be fair. It was a challenge from both of them," Bilic told Sky Sports.
"It was the other way round in terms of who did the more dangerous tackle. It was Jones. Okay he's carrying the ball but he went in with the scissors.
"Phil made a meal of it, but you can't blame him doing that. Maybe he made a meal of it because he went with the dangerous one to save himself.
"You have to see who is doing our challenge. It's Feghouli, who very rarely makes a foul in the game. His foot wasn't high in the air and it wasn't deliberate.
"For me, that was the crucial decision in the game. That early - it killed us. It put the game in a totally different perspective.
"That decision and the other one [offside goal] were totally unfair to us and totally wrong."
The other decision Bilic was referring to came for United's second goal, with Zlatan Ibrahimovic striking home despite being in an offside position.
"That offside is a big offside," Bilic added. "The players are not sprinting. It can be very hard if they're sprinting - a lot of times we need slow motion.
"But the players were walking. Good referees should have noticed that it was offside."
Bilic admitted that he could not fault West Ham's performance, with David De Gea putting in a terrific save to deny the hosts from taking the lead in the first half.
"I'm pleased with the performance. We fought really hard and gave everything. If we continue to do his we will be fine in the table," Bilic said.
"I'm gutted about the result. With 10 men for that long, against a really good team, it's very difficult. But we defended really well.
"When it was still 0-0 we had the chance of the game. It's these moments when you have to score. To get that lift and strength."