Callum McGregor branded Celtic's 6-0 defeat to Atletico Madrid as "embarrassing" after their thrashing in Spain.
Doubles from Antoine Griezmann and Alvaro Morata, as well as strikes from Samuel Lino and Saul Niguez, saw Diego Simeone's side run out comfortable winners at the Civitas Metropolitan Stadium.
A red card early in the first half for Daizen Maeda changed the course of the game for the Scottish champions and McGregor insists Celtic must learn quickly from their mistakes.
Speaking to TNT Sports, he said: "It was a very tough night for sure. It is embarrassing to lose six goals and we have to learn quick.
"That has to be a one off, we have now come to two away games and had men sent off. This level is difficult enough when it is 11 v 11and that is not to point fingers at anyone because I think all three of the challenges have not had any malice in them.
"We have to learn from that as this level is too difficult to play against teams with 10 men or nine men and you see the massive quality with their goals as they scored with almost every shot they hit.
"I thought we started the game quite well and even when we lost the goal, I thought we were decent value and in the game.
"If we then get to half time at 1-0 then game probably feels a bit different for us but then we make the match so difficult for ourselves to be fair to them that was a clinic on how to play against 10 men.
"They really stretched the pitch, and you see how they got in time and time again."
