Stabler and Namath never let their partying interfere with their game on the field. Manziel never lets the game on the field interfere with his partying. That's the difference.
I seem to remember a bleary eyed Namath trying to play a few downs and then puking his guts out on the sidelines. TV announcer said he apparently had a "stomach flu" but you could figure out what "Broadway Joe" had been doing Saturday night.
The NFL was a much different place 40+ years ago.