Pack covers are OK for hiking, but when you're zipping along on a bike they turn into a drag chute (which fills up with water at the bottom) no matter how well they're fitted to the pack.

Don't worry about the bag getting wet, worry about the contents. Get a dry bag and use it for a pack liner. Venture Sports and Sea-to-Summit are the two best I've seen. Venture Sports dry bags are vinyl, so they're heavier and more puncture/abuse resistant. S-t-S bags are SilNylon and very lightweight, but are more prone to getting torn if you overstuff them or carry sharp things.