"I never thought that I am so loved," Iordache said on Twitter after an outpouring of social media support from fellow competitors, including Biles.
Iordache, 21, left the arena in a wheelchair.
The International Gymnastics Federation, citing the Romanian Federation, later confirmed the injury was a torn Achilles.
It is the second successive year that injury has kept Iordache out of a major competition.
After claiming a bronze medal in the team event at the 2012 London Olympics, she missed last year's Rio Olympics due to a broken finger.
In Iordache's absence on Wednesday, Japan's Mai Murakami led qualifiers for Friday's all-around final, ahead of American Ragan Smith and Canadian Elsabeth Black.
The men's competition in Montreal had already lost major drawcard Japan's Kohei Uchimura.
Uchimura, who has won the last six world and two Olympic all-around titles, pulled out of qualification on Monday with an ankle injury.
The championships continue on Thursday with the men's all-around final. Cuban Manrique Larduet was the top qualifier.
(Reporting by Andrew Both in Cary, North Carolina; Editing by Greg Stutchbury)
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