Deila's first loss as Celtic manager saw his team thrashed by the Polish side in the third qualifying round first leg of the Champions League.
Legia Warsaw are managed by another Norwegian, Henning Berg. The battle of the Norwegian bosses gave a surprise victor.
Berg said after the tie: “I am proud of my team. It is great to see a Polish team play like this in such an important match. It was a great evening, and I am happy to be able to smile again.”
The Scottish side took an early lead after 9 minutes through Callum McGregor. But Legia quickly replied with two goals before half-time from Miroslav Radovic.
Celtic began to fall apart in a disastrous display that saw Efe Ambrose sent off in the 43rd minute, two penalties conceded and missed by the Polish team, before Michal Zyro and Jakub Kosecki scored near the end of the game.
Deila said after the match to NTB: “I am terribly, terribly disappointed. We started well and got the first goal, but then we started losing the ball easily. We gave away chances. I am disappointed that we did not keep together as a team.”
But the Norwegian boss can prepare for a backlash from fans and the media.
UK journalist Liam McLeod said about the match: “Ronny Deila will get a terribly tough week with very hard criticism. We are shocked by the way Celtic lost this match and by how poorly they defended themselves. I cannot remember more than two matches in the last years where Celtic showed to be so weak in defence.”
“From being in heaven, Deila now goes straight to the basement. He now has to build everything up again. The loss becomes worse when Celtic cannot play the return match on their own home field.”
Celtic must win at least 3-0 to overcome this loss and progress to the Champions League.