Haddin batted in different positions and in Sunday's final against England [ Images ] he was sent at number four, where he struggled to adjust.
"The Aussies did not make any mistake in the entire tour. They just failed to perform in a crunch situation. The only area where they went wrong was using Brad Haddin as a floater," said the former Australian pacer.
"Haddin wasn't entirely sure of his role and struggled in the final, not really knowing what to do and when to hit out," Gillespie told ESPN-Star.com.
The failure of the top-order was also instrumental in Australia's defeat in the final, according to Gillespie.
"Shane Watson [ Images ] and David Warner were the form guys for Australia and when they went early, the team was always going to face the heat," he said.
Gillespie was all praise for England, who won their first ever ICC [ Images ] title.
"England were brilliant on the field. They were positive right from the start and it showed how confident each player was about his role in the team.
"The best thing about England's campaign was the way their youngsters performed in the middle. Craig Kieswetter and Michael Lumb were magnificent and the others chipped in at the middle order.
"They played to their skills and did not show any nerves. That for me was the key. They deserved to win the ICC World Twenty20," he said.