1) The Boston Celtics and Miami had good players, so after pre-camp and a full (82 games long) season they've been able to get to the place they got.
BUT, as I already wrote...
2) Miami started very poorly, as they had no "team-chemistry".
3) It is not by chance that the phrase "team-chemistry" is something regulary used in sports.
4) A player who is part of his team for more time knows the plays better, knows how the strengths and weakness of its teamates, etc.
This should be affected in the Game-Engine.
5) How should it be implented is not a problem.
There are several ways to handle it.
For example, not allowing that player to have more than certain Game-Shape for few weeks.
Another one - lowering his current status (Passing, Defence etc.) for few weeks by few points.
etc.