I don't understand how being silent when you are severely dissatisfied at a sporting event does anything but send a wrong signal.
The Garden crowd doesn't boo every game. When the Celtics are giving a good effort, win or lose, you never hear boos. You don't hear cheers in a game when the team is playing well and boos when they aren't. The only time the Garden rains boos down on their team is when they are giving no effort and severely under performing because of it. And usually the crowd will actually cut this team slack for game after game after game, week after week of under performing before they finally start booing.
The crowd has been deathly quiet during some great stretches of Celtic basketball over the last two years. Sometimes the only time they get loud is when the scoreboard is imploring them to cheer. It's embarrassing. So if you get deathly quiet when you are not happy, how is anyone supposed to differentiate you from when the team is playing well because most crowds over the last two years have been deathly quiet during great performances.
You pay your money and have every right to voice your opinion of the way the team is performing any way you want. Cheer, be quiet, boo. I understand the cheering and think that's what people should be doing all the time. Leaving a game you should barely have a voice, IMO. After games and games and games and games of horrible effort and play, I don't see how booing to show dissatisfaction and non acceptance of that play is a bad thing. And I don't see how being quiet during those specific times makes any sense.
You're mad. You're not happy with what you are receiving from the team you paid to see.
Boo loudly. Boo your lungs off. You should let management, the coaches and the team know you're not happy. Otherwise, if you keep quiet, they just might think that the crap they are giving you is acceptable, and it's not.