Ronny Deila played down talk of new signings in January despite seeing Celtic fall to a 4-3 defeat away to Dinamo Zagreb in the Europe League.
Kris Commons and Stefan Scepovic had Celtic briefly ahead, but a Marko Pjaca hat-trick and Marcelo Brozovic strike meant another four goals were conceded by Celtic's shaky defence.
However, Deila backed his defence.
"I think we have players to play a good defence," said Deila. "I don't think that's the main problem."
Celtic went into the game knowing they had already qualified for the last 32 whatever the result and Deila criticised the desire of his makeshift team.
"It was a poor performance, we were not 100%, we lacked the intensity and aggression we had against Motherwell," said the Celtic manager. "We were not 100% up for the game.
"Even though we scored three goals and I think the subs were very positive when they came on ,defensively we gave away too many chances and conceded four goals.
"You should win when you score three goals away in Europe.
|Celtic manager Ronny Deila|
|"We had an opportunity to win the game when we go 2-1 up and that's disappointing when we give the victory away like we did."|
"We had an opportunity to win the game when we go 2-1 up and that's disappointing when we give the victory away like we did."
Among those entering the competition for Monday's draw will be teams dropping out of the Champions League group stage and Deila realises his side must improve against such opponents.
"What we have to think about is what kind of formation we will play and how we play when we play away in Europe," he said.
"This game we were more offensive than maybe we should have been.
"The team spirit has to be much better and we need to be more compact as a team.
"Away in Europe, you really need to be compact and aggressive and you have to keep concentration for 90 minutes and, if you don't, you will get punished like we did today."
Celtic could bolster their squad ahead of the next stage by signing players during the January transfer window.
"If we get players that can improve us as a squad and as a team we will look for it, but it has to be the right players," added Deila.