Denmark believe their World Cup Group C encounter with Australia in Samara on Thursday will be won or lost in the minds of the players.
"We forgot that."
The 64-year-old expects his team to be better prepared for the clash with Australia.
"Mentally you have to be strong. That is the all-important point," said the Norwegian.
His thoughts were echoed by his players.
"The mental aspect will decide the game," said Yussuf Poulsen, who scored the winner against Peru. "That is always the case. If you are mentally 'there' you will be willing to go the extra mile and that will decide it."
Goalkeeper Kasper Schmeichel added: "Mentally you have to be ready to fight for it as it is the small margins that make a difference at this level."
Schmeichel has not conceded an international goal for 534 minutes, breaking the Denmark record held by his father Peter.
Peru rained down 17 shots on Schmeichel's goal during their defeat but Hareide was confident that his defence could remain fundamentally sound and that there would be no repeat.
"Of the last 16 matches we have played, it is the match where we have conceded the most chances against us."
Denmark conceded nine goals in 12 games during their World Cup qualifying campaign.
Denmark will qualify for the last 16 if they beat Australia and Peru fail to win against France.
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