Walter Mazzarri was critical of referee Graham Scott after 10-man Watford were beaten 3-1 at West Brom on Saturday.
The visitors had the opportunity to move into the Premier League's top six with a win, but instead it was Tony Pulis' men who climbed the table as they secured a hard-fought victory.
However, with Watford dominating throughout, Mazzarri was frustrated to see a number of decisions going against his team – including a late red card for Roberto Pereyra, who clashed with James McClean, before West Brom's third goal.
"The sending off was unfair," said the Italian. "If someone had to be sent off, it should have been McClean.
"We played the best football in an away game since I arrived here. We did not deserve to lose.
"We played a great game that was influenced by the referee."
Pulis, however, agreed with Scott's interpretation of the incident as Pereyra appeared to kick out at McClean before a confrontation.
"I was more concerned about James McClean," the West Brom boss told BBC Sport. "Trouble seems to follow him around. I was more concerned with keeping him out of trouble.
"The lad [Pereyra] has kicked out. Whether you catch him or not, it is the intent. "
