The Netherlands international, whose Ajax team travel to Madrid on Tuesday looking to overcome a 2-1 deficit, said Barca had urged him to help inflict a defeat on their oldest rivals.
“I spoke to them when I signed the contract and they told me to try to knock out Real Madrid, as that would be great for Barca too,” he said.
De Jong urged Ajax to be courageous and said he believed they could cause a shock by eliminating Madrid, who have lost their last three home games.
“We have to play on the attack, put them under pressure and play great football,” he said.
“If everyone contributes, we have enough quality to cause a surprise.”
Madrid captain Sergio Ramos is suspended on Tuesday because of an accumulation of yellow cards.
“Ramos is Madrid’s boss on the field and in the backline,” Ajax coach Erik ten Hag said on Monday.
“His absence clearly is a loss tactically, but also mentally. It’s a loss that will clearly be felt by Real Madrid. It obviously won’t be as good as a team as if it had Ramos on the field.”