One reason the Celtics always seem to have a chance is that they call the best set plays in the NBA. They have a great coach who draws up things other coaches never would, and they have players who can execute those plays. In particular, the Celtics run great misdirection plays, sending all the action on one side, then sneaking a star out for an open shot on the other.
Those plays killed the Heat in the second quarter, and they allowed Paul Pierce in particular to get open looks. Misdirection is particularly effective against the Heat because of the way they pressure the ball. It’s now on the Heat to adjust.