I know a player is a Celtic when that player embraces the team, commits to the team by his actions. You know the player is a Celtics when there is no doubt in your mind that when that if that player is treated well by the organization he will not leave the team. That player may be dealt away in a trade, but that trade will either sadden him or he will do it for the better of the team.
Going by the above definition IT was the or is the ultimate Celtics. He loved the organization and played his heart out for the team. Larry Bird, Kevin McHale, Robert Parrish, Late Dennis Johnson, Paul Pierce, KG, Perkins, Bill Walton, to name recent ones were all the true embodiment and fit the definition.
I look at this team so far, and I see perhaps Horford and Smart. It is tougher to define the lesser players that to a large extent do not control their own destinies, but for those that do, it is easy to see who is and who is not a Celtic.
It could be one of those things that cannot be easily defined, but you know it when you see it. While it may not be easy to tell when a player is a Celtics, it may be easier to tell when one is NOT.