Easily, it is the death of Len Bias.
Red had once again displayed his genius by building the 80's team in the first place and then acquiring the #2 pick in the draft in the middle of a prime title run. Bias' loss, along with Reggie Lewis later, was the one-two punch that ended the Celtics stranglehold on the NBA. Of course, David Stern and the league owners blocked any fair compensation for the losses the Celtics suffered with Bias and Lewis. Took years to recover from those tragedies along with the insane hiring of Rick "Ego-Maniac" Pitino, who caused untold damage to the franchise all by himself.
A Side Note: What if Kevin McHale doesn't lose the Laker missed free throw rebound out of bounds that led to the infamous Magic Johnson hook shot across the lane that won Los Angeles Game 4 in the Garden in the 1987 finals ? Celtics tie the series @ 2-2 with game 5 in Boston instead of being down 3-1. Nobody ever talks about McHale's complete screw-up of not boxing out and allowing the Lakers to pressure what should have been a routine defensive rebound (I believe it has become accepted fact that a Laker actually touched that ball last and if automatic reviews had been in place then, the Celts would have correctly been awarded possession). Celtics did go on to win game 5 and, hobbled with injuries as they were, would have had two shots at the title back in LA.