[Adam]

Blindzebra nails it. In this case, if I see the kid fall when the ankle didn't twist, I'm letting it go. I often let the play take shape before whistling the play dead on a defensive injury anyway. If the player is in peril, I'll likely err on the side of safety. If he falls in the middle of the crowd, and I'm convinced he's taking a dive, I'm hitting him with a T.
If I'm not sure, and he's in the middle of the play (somewhat likely given the likely full court press, unless his teammates stand around doing nothing), I'll whistle it dead.