Mikael Lustig admitted Celtic had been "all over the place" at the back after going down to a disappointing 2-1 Europa League defeat against Molde.
The Norwegians secured their place in the last 32 of the tournament by completing the double over Celtic, who now sit bottom of Group A with just two points to their name.
Goals from Mohamed Elyounoussi and Daniel Berg Hestad were sandwiched in between a deflected effort from Kris Commons during an eventful first-half in Glasgow.
Celtic - who finished the match with 10 men following the dismissal of Nir Bitton with 13 minutes to play - have now conceded nine goals in four European games this season.
"Overall I think we probably created a few more chances than them, but the two goals we've let in are not good enough," Lustig told BT Sport.
"If you look against Fenerbahce we made a quite decent performance as a team on defence. It was maybe individual mistakes.
"But today, I think we were all over the place. A team like Molde, we shouldn't let them have so many chances."
Despite the disappointing result, Celtic can still qualify for the knockout stages of the competition.
They face Ajax at home next before finishing their group fixtures with a trip to Turkey to play Fenerbahce.
