Mikael Lustig conceded Celtic's performances in the UEFA Champions League were not good enough after crashing out of Europe.
Celtic saw their European hopes for the season ended after a disappointing 3-0 defeat to AC Milan in Glasgow.
The Scottish champions were guilty of giving Milan a helping hand to victory with some slack defending allowing Kaka and Cristian Zapata to score from two corners with Mario Balotelli wrapping the game up for the Italians.
Lustig was unhappy with the nature of Celtic's defending from set-pieces as they gifted Milan their opening two goals.
"I don't think we've let one goal in from set-pieces this season and now we do it in the most important game of the season," Lustig told Sky Sports.
"It's not good enough.
"We've conceded two or three goals from corners in two years and I don't think it's the formation that's wrong.
"The first goal is a free header from about two or three yards out."
Lustig admitted a lack of goals in the Celtic squad, having scored only two in five group-stage games, was to blame for their early exit from Europe.
"We've played five games in the group and scored in just one game so I think it's not good enough," Lustig added.
"We were hoping to get through to the last 16 and if not that then we were hoping to get into the Europa League but it's not so."