Dundee United manager Peter Houston says Robbie Neilson’s red card cost his team the chance of a Scottish Cup win against Celtic.
The Scotland assistant manager said he hadn’t seen the incident which led to the defender receiving a second yellow card.
However Houston expressed his disappointment that the former Hearts and Leicester City man had felt the need to make the challenge in the first place.
"I feel it's hard enough trying to beat the champions-elect with 11 players never mind 10," he said.
"I haven't seen it yet but what I would say is that Robbie is on a yellow card and it's a challenge that he makes inside Celtic's half.
"I can understand if it was around the penalty box and they were going to score.
"He is not a dirty player but he is an experienced enough guy, he should back off and not go into a challenge like that.
"It was a pivotal moment because effectively, it had a bearing on the game, rather than us being able to continue that good start we had, we had to take off a young flair player in Gary Mackay-Steven."
Celtic midfielder Joe Ledley was in no doubt that referee Ian Brines had made the right decision.
He said: "It was a definite sending off.
"He has led with his elbow and caught Georgios on the nose, even the player knew himself it was a sending off and so the referee got it right.
"They caused us problems in the first 15 minutes and they looked the better team and then the sending off did kill them and after that we dominated.
"The manager wasn't happy at the way we were playing in the first half and he made it clear in the team talk at half-time and we looked a totally different team in the second half and it was nice to get on the scoresheet."