Ray Allen ultimately chose playing time over loyalty and no one, Celtics fans included, should blame him. Allen was the third wheel in the Big 3 marriage from the start, as he had the most difficult time adjusting from being THE MAN in Seattle for the Supersonics to the third wheel (and more recently, the fourth or fifth wheel) in Boston.
Miami simply wanted Allen more. The Celtics, who had tried to ship Allen out of town on multiple occasions, hadn't REALLY wanted the NBA's all-time 3-point shooting leader for some time now. In the past week, the Heat wooed Allen like no one since Jim Calhoun way back in the early '90s when Allen was a wide-eyed teenager. Miami promised him the world, except more money.
Whenever an athlete takes less cash, he is typically applauded. Not Allen.
No. 20 will be booed every single time he touches the ball in Boston as the perception is that he joined the NBA's evil empire just for the sake of wide open shots and the promise of another ring. Of course, rational minds shouldn't boo him. Allen is arguably the classiest player in the league.
Don't expect Allen to badmouth Boston. Don't expect him to leak any dirty details about Rajon Rondo. And we shouldn't expect the Celtics to go all Red Sox on Allen either.
Miami promised Allen the world. The Celtics did nothing of the sort. And that's fine, by both sides.
The Celtics will give Avery Bradley the playing time he rightfully deserves. They will give the newly acquired Jason Terry (not a bad shooter in his own right) the playing time he rightfully deserves. Even without Allen, they promise to be the deepest team in the league (as long as they sign Jeff Green). Boston will be fine.
Who knows what will happen with Allen? He could very well break down again. He could very well fade in the playoffs again. Or he could very well add a lot to his legacy as the greatest 3-point shooter in NBA history. It just won't be as a Celtic. And that's fine. He was a mercenary from the start, remember.