The Norwegian's underwhelming start to life at Celtic Park worsened against Legia Warsaw as they were beaten 2-0 in Glasgow to bow out of the competition 6-1 on aggregate in the third qualifying round.
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"I have only been here for six weeks, but right now it's not good enough. That's what I can see," Deila told reporters.
"We need to make the squad better by getting new players, which we are working on. But we also need to get more from the players here and there is a lot of quality.
"They just haven’t shown it in the last two games but that’s my responsibility. I will need to build up a new team. That’s two years in the Champions League for Celtic but this year we're not good enough by far.
"We must do everything we can to make it different next year. We have to bring the best quality in now with the money that we have. I know that the club wants to improve the squad."