Clearly they will regret booing him. I feel terrible for him, but you need to look at it from the fans perspective.
They're stitting at the game (paying full price for a preseason game which is a different topic for a different day), and they find out via Adam Shefter's Twitter that the star quarterback who is supposed to bring them to a Super Bowl (hell this year could have been their year) is gonna retire 2 weeks before the start of the season. They see him standing on the sideline chatting with his teammates. What reaction should they have? Cheer the guy who (lets be honest here) is screwing over their season?
Plus it isn't like the injuries were career ending injuries. It was just continuously having different injuries taking the mans will to play. If he was unable to come back from the shoulder thing the fans in attendance would likely understand more, but he did come back from that.
It is really terrible. What's worse is that my first reaction (after the shock) was to add Jacoby Brissett to my fantasy team.