Karpov would have totally destroyed Fischer. Fischer had no positional knowledge
Ateatoenail Karpov loses in 12 moves to Larry Christiansen
http://www.chessgames.com/perl/chessgame?gid=1069116
Someone who blunders as badly as Karpov would have been Taimanov'd by Fischer.
Karpov has said himself that he only gave himself a 40% chance of winning against Fischer. That does not bode well when you have to play against the greatest player the world had ever seen.