I believe autophagy doesn't kick in for the first 5 days, so if you want to preserve all muscle mass, that's the window to stay under. Once you go after that, the changes aren't that dramatic either, so you'll recover whatever muscle loss you incur fairly quickly as long as you're fasting while still having fat reserves. Once you're out of those, fasting is a bad idea, in the sense that you will start dying.
It's not like your body is going to eat its own muscle! It's not that stupid. The muscle loss is due to the normal wear and tear of daily activities. If you train hard during a prolonged fast, it's likely going to lead to more muscle breakdown. And autophagy is not that either. Also, it kicks in much sooner than 5 days.
As for fat reserves, even fairly lean individuals can probably go a week or two without food without breaking down much muscle. 14 days at 2500 calories is merely 10 pounds of fat. Unless you are morbidly skinny, you should have 10-15 pounds of fat ready.
https://www.amazon.com/Complete-Guide-Fasting-Intermittent-A... - another book by Jason Fung that covers water fasting more in depth.