When you are a coordinator brought in to save the HC, you are in a bad spot. First, even if you do a good job, it could fail and the HC still be fired or else on a very hot seat: see SC this year.
But if you succeed very well, then some of those fans who'd demanded that the HC be fired or else replace at least 1 coordinator will then want you to replace the HC you saved.
Harrell is not a Longhorn - he's a small town West Texas Red Raider. He should be very wary of going to try to save Tom Herman.