Bardhan recalled that he had earlier cautioned over the "arrogance" of Left leaders, but refrained from blaming anybody in particular this time.
"Remain a Communist but not a dogmatic...Those who cannot change will be out. Either you change or you are out," Bardhan told a news channel in an interview.
Bardhan also advised the Left to be prepared to have a "long stint" in the opposition benches and be a "constructive, responsible" opposition which does not enter into confrontation with the government every day on all issues in West Bengal.
"Accept the defeat with all humility. It is a big setback. Be prepared to have a long stint in the opposition. It will be for the benefit of the Left Front. Be constructive, responsible opposition. Don't enter into a confrontation everyday on all issues," he said.
"Don't try to be non-cooperative. Trinamool Congress was non-cooperative. We should cooperate if good things are done. We should support it. But in the name of change, if all gains made earlier are reversed, then we need to fight," he said.
The CPI leader said he expected that the Left would lose West Bengal but the extent of the defeat was a "surprise" and "shock". Asked about the results, he said, it was a "defining moment" which makes one sit up and reflect.
On whether it was the arrogance of Left leaders which led to the electoral setback, he noted that sweeping victories make one "hot-headed, a bit arrogant" and makes "you feel you can take people for granted".
"There have been omissions and commissions of Left. The behaviour of Left cadre was on. They were the vote in anger against the Left. They (Singur and Nandigram) triggered off the anger.
"It gave a feeling Left which once looked invincible can be fought and defeated. Till then a section of the anti- Left did not believe that Left could be defeated," he said.
Asked whether the decision to withdraw support to UPA-I on the nuclear deal resulted in Trinamool and Congress coming together, he said a section thinks so and added that nuclear deal was the issue to break the relations though snapping the ties was "inevitable".
He said there was no point in blaming West Bengal Chief Minister Buddhadeb Bhattacharjee alone for the debacle though the captain has to take responsibility.
Noting that VS Achutanandan was the most popular in Kerala, he said nothing went wrong in Kerala and he expected the Left to win there.
Bardhan said: "Achutanandan was the most popular man. It was he who was the crowd puller. He pulled more crowds than all who went from here to address meetings there."
Asked whether he thought that the Left leadership should step down owing moral responsibility, he said such things do not solve the problems for a party as it would not help in reflecting on the shortcomings.
"It is the easiest thing to do. It only helps in deflecting criticism," he said.
However, he said he would step down at the next CPI Party Congress as age was not on his side.
He said the Left leaders should go back to people "not as rulers, not as those who decides the destiny of the people." Bardhan was of the view that the Left needs to have a base in Hindi-belt.