Briton Cavendish, the world champion, is regarded as one of the favourites for gold in front of his home fans having won the last two sprints of the Tour de France in awe-inspiring fashion this month.
Irishman McQuaid, a former professional rider, picked the 250-km course himself and thinks the British team should expect some tough competition, especially from Belgium.
"Cavendish, I don't know. 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, I used Box Hill a lot as a student," McQuaid told Reuters at a central London hotel on Thursday.
"For the event and for the Olympics, obviously Cavendish would be a great winner. It would definitely lift the spirits.
"(But) Cavendish...that's not a given."
The Manx man looked way ahead of his sprint rivals at the end of the Tour but even after losing four kilogrammes in order to improve his climbing abilities, he should not expect to glide up Box Hill.
The hill to the south west of London is a short, steep climb the riders will tackle nine times.
"The circuit is hard, the hill itself is not that hard but you hit it in quick succession," McQuaid explained.
"When you're over the hill, then you drop down rapidly and you're back up again. Down and back up again."
McQuaid recalled a chat he had with Italy team coach Paolo Bettini after last year's test event on the course.
"After the test event finished, Bettini came to me and he said it is not going to be as easy as people think next year. It's very much like a Belgium classic. These are small roads, up and down," he said.
No wonder then that McQuaid tipped Belgian Philippe Gilbert, a one-day race specialist, as a possible winner on Saturday.
"It could suit Gilbert, and Tom Boonen if he is in good form could be up there. And if it splits or a breakaway gets away it could be anything," he said.
The very nature of the course could leave Britain with the responsibility of controlling a race in which radio communications between the coaches and riders are banned.
"The British may be left to control the race alone. And the only ones they are going to get help from is Germany (who will look to set up a sprint for Andre Greipel).
"And 250 kilometres controlling the race is a lot."
McQuaid is expecting the road race to draw a record attendance for an Olympic event.
"Last year when Cavendish won the test event, there were 400,000 people. Since then he has become world champion and has proven to be a good world champion," he said, adding that Briton Bradley Wiggins's recent Tour de France triumph was also a factor.
"With all of that success, I predict a million people on Saturday, which would be the biggest crowd ever for an Olympic event, I imagine."