Gerbeau said that pressure on his time as chief executive of leisure company Xscape would rule out any involvement in the Olympic bid.
He said: "Working on what's now part of a billion-pound business, I'm afraid I couldn't commit the time required."
Gerbeau added that Cassani was the right choice for the job but confirmed that he had been approached. He said: "Barbara is actually a great friend of mine and is perfectly capable and suited for the job. I did talk to a few people about the post."
The former Olympic ice hockey player has been applauded for his work at the Millennium Dome. He became chief executive of the dome's parent, the New Millennium Experience Company, in 2000. Gerbeau was considered to have fought a brave battle in trying to save what had already become a financial and critical flop.
Gerbeau suggested earlier this year that the dome would be ideal for Olympic events such as boxing and show jumping.