The Danes failed to set pulses racing in the opening round, featuring in the first goalless draw of the World Cup against France, a result that ensured both sides progressed from Group C.
By contrast, Croatia, stormed through Group D, winning all three games. They also put in arguably the best performance of the first stage, a 3-0 thumping of Argentina, in Nizhny Novgorod, where they will also play Denmark.
But Hareide said his team would be more expansive and play differently on Sunday.
"I think you will see a different Denmark from what you have seen so far and the type of game we have played," Hareide said at a press conference. "Yes, we need to defend but we need to attack more."
He said the match could end up being decided by two opposing midfielders, Modric, one of the players of the tournament so far, and Denmark's highly-rated Eriksen.
"It could be a match between Modric and Eriksen, that would be interesting, that would be great for the fans."
Denmark are unbeaten for 18 games, a national record, their last defeat coming against Montenegro in October 2016. They have also kept six clean sheets in their past seven games.
If Denmark beat Croatia, it will be only the second time the Danes have qualified for the last eight of the World Cup, the other occasion being in 1998.
Later, at Croatia's press conference, Luka Modric would not respond to reported remarks in the Danish press attributed to Eriksen that "I don't consider myself to be worse than him (Modric)."
The Croat captain said: "I don't like to compare players, to compare others to myself. Eriksen is a fantastic player and he's had a fantastic season at Tottenham. He is the most important player for Denmark."
Croatia's coach Zlatko Dalic said his team were under no extra pressure as they suddenly found themselves predicted by many to win the World Cup after their group performances.
"We have no pressure coming from the outside, our greatest pressure comes from ourselves," he said. "We've come to perform well, to achieve something. In the group stage we played well but it will mean nothing if we do not verify that tomorrow."
The two teams have only met in the finals of a major competition once before, when Croatia won 3-0 at Euro '96, and Davor Suker famously lobbed Danish goalkeeper Peter Schmeichel.
His son, Kasper, is now the Danish 'keeper and, despite saying earlier this tournament he was tired of answering questions about his father, the younger Schmeichel was asked if he knew who Suker was.
"He's a Croat, that's all I know," he deadpanned.