It appears, through game play, that once starters are subbed out of the game, their return depends most, in most situations, on the sub becoming fatigued.
Observed behavior: Kobe Bryant subbed out of the game. Kobe Bryant returns to near 100% on fatigue bar, i.e. not fatigued. Kobe Bryant not auto subbed back into the game until replacement/subbed player fatigue falls.
Often a starter, like Bryant, sits out the last two minutes of a half or game because the player subbed in for him isn't yet tired.
Expected behavior: Kobe subbed out of game. Kobe returns to near 100%, or even to 75%. Kobe returned to game regardless of the fatigue of the replacement player.
Particularly for stars, most often NBA coaches don't want them sitting on the bench when they're rested.
Particularly down the stretch in tight games, stars should not sit on the bench while a non-tired sub plays during the last two minutes of the game.