Been around boxing, both in and out of the ring, for most of my life, so I know a good amount about the sport. The term "puncher's chance" is reserved for fighters who are severely outmatched by their opponent, but still have enough power to KO them at any given moment.
Mcgregor has the power and speed to land a devastating knock out blow if Mayweather makes a mistake or just doesn't respect his power(ala Aldo). Yes Aldo is nowhere near on Mayweathers level, but is a professional fighter and had a gameplan that was ended in the opening seconds of their fight. Mayweathers been rocked more then once, so it's not impossible. As I said before, I dislike them both and wish they'd both lose somehow, but to say Mcgregor doesn't have a punchers chance is silly, IMO.