The Scottish giants were knocked out of Europe's elite club competition for the second time this season Tuesday after defeat at home to Maribor.
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The defeat at Celtic Park saw the Scottish champion dumped out of the competition for a second time this season, having only been reinstated following Legia Warsaw's punishment for fielding an ineligible player.
Maribor's 2-1 aggregate triumph was sealed by a Marcos Tavares strike 15 minutes from time Tuesday and Deila, whose side also lost to Inverness Caledonian Thistle in the Scottish Premiership on Saturday, offered no excuses for failing to reach the group stages.
"They got two chances during the game and they were more effective than us, but in the end there is only one thing to say and that is we weren't good enough and we didn't deserve to go to the Champions League," he told the club's official website. "The pressure was too little and there was not enough aggression, but in the second half we were more aggressive and we got some chances as well and it was an even game.
"We were ready to go and we wanted to go hard and pressure them but I think, again, we were a little bit frightened and wanted to protect too much."
Celtic will now go into the draw for the Europa League, which takes place Friday.