"Even now I believe in his innocence. He has always respected all the rules," Indurain, who won the Tour from 1991-95, was quoted as telling Radio Marca.
"I'm a bit surprised. It's a bit strange that this was only based on testimonies," he added in reference to USADA's 1,000-page report featuring testimonies from 11 former Armstrong team mates.
Armstrong, 41, was formally stripped of his seven Tour titles on Monday when the International Cycling Union (UCI) ratified USADA's decision to ban him for life and nullify his results since 1998.