The Dutch were amongst the favourites heading into the tournament but lost all three matches and were eliminated after finishing bottom of Group B.
De Boer admitted he was extremely disappointed with the Oranje's poor showing in Poland and Ukraine but was unable to put a finger on what exactly went wrong for Bert van Marwijk’s men.
“Just like the other 16 million people in Holland, I found it very disappointing,” the Ajax boss told reporters on Wednesday.
“Was there a difference between now and two years ago? I don't know, but something wasn't right.”
De Boer was an assistant to Van Marwijk during the 2010 World Cup, when the Dutch lost in extra-time in the final to Spain, but dismissed suggestions that his absence had any effect on the team.
“You can't say that,” he insisted. “Of course, I've had my share, just like all the other members, but I'm sure Philip [Cocu, current assistant to Van Marwijk] will always gives his opinion.”
De Boer also admitted it was unsurprising that many observers believe Rafael van der Vaart and Klaas-Jan Huntelaar should not have had such peripheral roles for the Dutch.
“Everyone is talking about it,” the former Netherlands international continued. “[Unlike] two years ago, Van der Vaart and Huntelaar are now key players in their club team.
“So I can image that if you're top scorer from the Bundesliga, you would like to perform on the highest level too.”