Robbie Neilson's sending off was described as a ''pivotal'' moment by his manager as Dundee United subsequently slumped to a 4-0 defeat yesterday.
There was both frustration and disappointment at the actions of the full-back who caught Georgios Samaras with his elbow in a midfield aerial challenge and was shown a straight red card by referee Iain Brines after 28 minutes. United, who had been causing Celtic problems, eventually leaked four goals as Neil Lennon's side strolled into the semi-finals of the William Hill Scottish Cup.
''He does not need to go for the header," admitted Scott Robertson, his Tannadice team-mate. "It is not as if Samaras is in on goal. I am sure he will apologise for it. I do no think anyone can stick up for him and say it is not a red card.
''It is one of these things that just seems to happen against the Old Firm."
Johnny Russell was sent off for butting Kirk Broadfoot of Rangers at Tannadice earlier in the season.
Robertson, though, said of Neilson, who joined United in November: ''Robbie has done well since he has been here and I don't want to criticise him. We have had eight clean sheets with him in the team.''
However, there was frustration that Neilson's action had effectively scuppered United's chances of going through to the last four of the cup to face either St Mirren or Hearts at Hampden.
''The scoreline was eventually convincing but Celtic fans would have been worried,'' said Robertson, who believed United had matched the visitors for most of the tie. The Tannadice club certainly had the better of the earlier exchanges and Milos Lazny and Russell had good chances in the second half to tie the match before Celtic strode away with the victory.
''We were putting Celtic under pressure. We were well in the game and obviously the red card has changed that because we have had to step back a little and hit on the counter attack,'' he said. ''Dusan Pernis does not have a save to make yet he loses four goals. They took their chances and we didn't.''
Robertson, who admitted a domestic treble now looked ''likely'' for Celtic, added: ''They have been the best team this season.''
His annoyance was also matched by that of Peter Houston, the United manager.
"I feel it's hard enough trying to beat the champions-elect with 11 players never mind 10," said Houston. "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. He has been rash. He went for a ball that he was not guaranteed to win.''
Houston added: ''It was a pivotal moment because 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."
Garry Kenneth was brought on to shore up the defence but in the second half Celtic scored four times through Joe Ledley, Samaras, Anthony Stokes and a Scott Brown penalty.
Houston, though, was consoled by the performance of his side, particularly in the early stages of the match. ''We were playing the best team in Scotland and we were in many ways on the front foot and dominating the game,'' he said. ''I am proud of the guys over the last number of weeks. The attitude of the team is spot on.''