Javi Gracia was unable to keep his emotions in check after the decision to send off Jose Holebas during Watford's 4-1 home defeat to West Ham on Sunday.
The result saw the Hammers finish tenth, one place above the Hornets, after a Mark Noble brace helped the visitors on their way to a convincing win at Vicarage Road.
Having been 2-0 down at half time, Gerard Deulofeu scored 11 seconds after the restart to give the hosts hope, but they were soon on the back foot after Holebas was given his marching orders two minutes later.
The red card now means the Greek defender will miss next week's FA Cup final, with Gracia clearly upset by the decision when speaking on the club's official website.
"I think in this moment I don't want to say what I am thinking about that, but I only say I can't believe a player will miss the final for that play," he said.
"For that contact, I can't believe it. Maybe it was a light foul, maybe a soft foul but I can't agree with that [decision]. It wasn't a foul. It wasn't a foul. It wasn't a yellow card and for sure it wasn't a red. After two days, they say it is possible it [other sendings off] were a mistake, but I don't want that.”
With the FA Cup showpiece against Manchester City to come, Gracia wasn't concerned by the fact his Watford side finished outside the top half in the Premier League, instead focusing on what his side must do to overcome the champions at Wembley.
“In my opinion, it not so important to finish 11th, 10th or ninth,” he added. “Obviously we prefer to finish as high as possible, but we have to know how we can improve our play to compete better in the next game, the final. I am sure my players know how important that game is. I have no doubt they will give their best.”