The defeat not only leaves third-placed Juve's hopes of qualifying for the knockout stages hanging by a thread but also threatens their chances of qualifying for the Europa League if they do not finish third in Group H.

Juve had never lost three of their first four Champions League group games and although club president Andrea Agnelli said he was 'ashamed' by their performance, he added that Allegri would remain in the job.

"He's right (to feel ashamed), it was the worst first half," Allegri told Sky Sport.

When asked if he would resign, he added: "No, I'm not going to step down, absolutely. Because it's a challenge now. When it gets harder, it's even more beautiful. We must come out of it with courage and desire."

The loss was Juve's fourth in six games in all competitions, which does not bode well ahead of Saturday's Turin derby at Torino. Juve are eighth in Serie A, 10 points behind leaders Napoli.

For Haifa, it was their first victory in a Champions League game since they beat Olympiakos Piraeus in the group stage in 2002.

"It's difficult to explain (what went wrong), we just need to be silent because we did not have a good performance from the character point of view," Allegri said.

"We need to get out of this situation and keep quiet. Our tactics lack heart and passion. We play too individually, we have to go back to being a team."