Slaven Bilic criticised referee Mike Dean's decision to send off Sofiane Feghouli after West Ham were beaten 2-0 by Manchester United.
Dean showed his fifth red card in 15 matches this season after just a quarter of an hour following Feghouli's tackle on Phil Jones.
It was a 50-50 challenge and although Jones came off worse Bilic felt the United defender had actually committed the more dangerous tackle.
But Dean stunned the London Stadium by dismissing Feghouli and leaving West Ham to play for 75 minutes with 10 men.
They were eventually beaten by second-half goals from Juan Mata and Zlatan Ibrahimovic, and the fact the latter was in an offside position did not improve Bilic's mood.
The Hammers boss said: "The red card was so early and it had a major impact on the game.
"I was very surprised. I'm the first one to say referees have a very difficult job but I wouldn't have been happy if it had been a yellow.
"At half-time I didn't want to see the replay but after the game I saw it and it gave me proof I'm right.
"The more times I've seen it it's the other way round. It was Jones who made the more dangerous tackle than Feghouli. Jones makes a scissor, he got the ball but it was dangerous.
"We will appeal. I expect at least to win the appeal, which will not make me totally happy but I would be extremely gutted if we have to lose Feghouli for three games.
"The stakes are so high that it is very frustrating that sometimes your whole season, and your job, depends on one or two decisions and the second goal is clearly offside."
Substitutes Mata and Marcus Rashford eventually swung the game in United's favour, the latter setting up the Spaniard for the 63rd-minute opener.
Ibrahimovic killed off the match when he swept in a loose ball, but Bilic was also left to rue a missed opportunity by Michail Antonio, who was clean through with the score 0-0 but shot too close to David de Gea.
Jose Mourinho refused to be drawn on the red card but admitted Antonio's miss was crucial.
The United boss said: "I think it was not a very good performance. It is very difficult to play football in these circumstances with 48 hours between matches.
"It's amazing for the fans around the world but it's very hard for the players. Not just hard for the team with 10, also the team with 11.
"If I'm honest, if Antonio scores and it's 1-0 I don't think we win the game. When you're tired and losing it's even more difficult."