Montpellier, who hadn't won for two months, swept to a bonus-point victory after Clermont played the final half hour with 14 men following a red card for backrow forward Judicael Cancoriet.
"We didn't make the effort to play the rugby we planned. It's not the attitude of a team that wants to be in the top six," said Iturria.
"It's the worst away game we've had all season. Against a team like us, going through tough times, you have to make an effort, fight for each other.
"We didn't do anything to win and this is not normal when it's a cut-throat match. We must respect the club, honour this shirt. We can throw everything in the rubbish bin."
Montpellier scored tries through New Zealand winger George Bridge, centre Thomas Darmon, hooker Vincent Giudicelli, South African scrum-half Cobus Reinach and finally from Australian international hooker Brandon Paenga-Amosa.
