Celtic manager Ronny Deila labelled the late challenge from Dundee United's Ryan McGowan on Liam Henderson in the Scottish Cup replay as "stupid".
McGowan was red carded near the end of Wednesday's 4-0 defeat, but United have appealed against the decision.
"It was a tough tackle, I don't understand why he is doing that," said Deila. "It was 3-0, and it's in the corner.
"It's a stupid tackle. When you come from behind it could be dangerous."
McGowan was sent off minutes after Anthony Stokes was dismissed for reacting to an apparent elbow from Paul Paton, bringing the red card tally to six for the three Celtic-Dundee United games in the past fortnight.
Deila said: "I don't feel anything extra when we play Dundee United. It's a good team and it's a very important game we play, as were the other three, and that's why it's so tense I think.
"Football is important, to win is important, but we have to protect each other as well. My players are sometimes doing hard tackles as well but you have to avoid the stupid ones."
He also disputed claims from United that the tackle was similar to the one made by Celtic captain Scott Brown in the 1-1 draw at Tannadice.
"It was not as it is a tackle from behind, and it is in the middle of the pitch," he said.
"At the side of the pitch nothing can happen there, and you know that he is angry before he does it. I do not agree with that."
His side face Dundee United on Saturday, the first time that Gary Mackay-Steven and Stuart Armstrong will be free to face their old club after moving to Celtic Park during the January transfer window, with Deila admitting his squad could suffer from tiredness as the campaign draws to a close.
He said: "We have a big squad and so many good players. I've said all the time we have to use our squad.
"It's tough for the national team players. I am so impressed with a lot of them that can just go on and go on.
"But, this is the lifestyle and they start to get used to it now. It's very important that they get that break in the summer.
"If you take Dundee United, we have maybe played double the amount of games they have played and that's a big difference.
"In Spain they have one or two weeks' [break] at Christmas. That's a positive for the players and the national team players. I think that can help British football."