Callum McGregor insists Celtic must learn from their mistakes after the defeat to Feyenoord in Rotterdam.
The Parkhead club lost 2-0 in their Champions League opener after goals from Calvin Stengs and Alireza Jahanbakhsh, with the visitors' lack of discipline proving costly.
Gustaf Lagerbielke and Odin Thiago Holm were sent off within minutes of each other to leave Brendan Rodgers's side with just eight outfield players.
And McGregor feels Celtic must recognise their mistakes to improve. Speaking to TNT Sports, he said: "It was a difficult game. I thought we started really well and then they got the goal just before the break and that undoes all the good work we did in the first half.
"Obviously to get two men sent off in quick succession takes the game away from you but I think the boys hung in there well.
"It can be easy to go three or four or five [down] at that point which is testament to the group.
"Hopefully, they are going off thinking we did not give up and we did not quit and there is another game to get them [Feyenoord] back at Celtic Park.
"We have to take the lesson of playing will 11 men at this level, it is hard enough with 11 players never mind when you go to nine so we have to learn from that.
"Like I say, we have five games to try and get ourselves out of the group but I think the most positive thing is how we started the game and did not give up when went to nine men.
"We were still trying to defend the box and hang in there. It was a disappointing night but we have five games to try and fix it."
