Unless either the big money donors and/or Danny White basically tell him he has to.
But I do think Heupel already knows what needs to be done and will do it.
The last few games, whenever Heup is asked about the defense; I am hearing more than a hint of anger and frustration that is abnormal for him.
Say whatever you will about the man; Heup hates losing probably more than we do.
He may have never fired an assistant before; but we have never had an assistant fail in such an obvious and highly visible way before.