I'm sure Doc out of respect to the veteran asked Sam if he wanted to get in for a couple of minutes of garbage time, and Sam said something like "don't worry about it."
I have no official clue on this, just speculation on my part.
I think if Sam was told to go in he would have refused anyway. He was having way, way, way too much fun with his buds KG and Sugar Ray on the bench courtside. Considering as soon as the game was over he wouldn't be a Celtic anymore, what could Doc have done? Made him run laps?