After seeing all of this play out, I think Lindsey said everything Millsap said he said minus the "black". Many times when we replay conversations in our heads that hurt our feelings, we alter the tone and sometimes even the words the other person used. I bet Lindsey said....
"If you say one more word, I'll cut your *** and send you back to Louisiana."
That's a harsh thing to hear. Those are direct words. I don't think there is anything racist about that statement, but maybe Millsap took the tone as such. Being raised in a family that spent a ton of time in Louisiana, there is an identity that comes from being from there. Much like if you replaced "send you back to Louisiana" with "send you back to Boston" for someone from that area. Lindsey's statement could be construed to be taking a shot at Louisiana or people from there or even black people from there if Millsap tried hard enough.
Overall, I just think Lindsey said something more direct than Millsap was expecting. I don't think he used race in his statement to Millsap. Had he, he should have been fired on the spot. And if he had a track record of saying things like that, it would have emerged well before this incident.