Cavendish won the final two sprint stages of this year's Tour de France and will enter Saturday's race as one of the favourites, but McQuaid said that expectation does not always equal success.
"Cavendish, I don't know," Reuters quotes McQuaid as saying. "The course is much trickier than it looks. The course has been selected by me.
"I went to college on the course in the early 1970s, I used to cycle around those roads quite a lot and I used Box Hill a lot as a student.
"For the event and for the Olympics, obviously Cavendish would be a great winner. It would definitely lift the spirits. [However] that's not a given."
Cavendish recently declared that he was not feeling pressure ahead of Saturday's race.