Dundee United boss Jackie McNamara felt Barry Douglas's red card in the defeat to Ross County was extremely harsh.
Referee Kevin Clancy dismissed Douglas for a 65th-minute sliding challenge on County goalscorer Ivan Sproule
McNamara told BBC Radio Scotland: "It's not a red card and it's frustrating.
"I didn't even think it was a yellow card. He's got the ball there and his momentum's meant he's followed through. But he's made very little contact with Ivan."
And McNamara added: "I'm really disappointed with that.
"Obviously Ivan's reaction doesn't help the situation. They [the officials] said it was serious foul play.
"We'd just made a few changes at that point and it upsets the whole rhythm of the team."
It is the first time United have lost to Ross County in the league, with the Dingwall side's only previous victory over them coming in a 1998 League Cup tie - when now County manager Derek Adams scored twice.
The game's only goal came via a Sproule header in the second half.
"I'm disappointed with the goal we lost," McNamara said.
"Just one ball through and we lost concentration and they've got a goal at the back post.
"I don't think there was much in the game although it got stretched a bit when we went down to 10 men."
United's best chance came in the first half when a trademark Jon Daly header, from Douglas's free-kick, was well-saved by home goalkeeper Michael Fraser.
They have now lost both of their top-six games since the Scottish Premier League split.
Their next match is against St Johnstone at Tannadice on 4 May.