Have you thought that maybe RLB was actually protecting Rigg when he subbed him against Forest? There was no way back from 0-4 at half time, and it was obvious that the crowd atmosphere was not going to be very positive in the second half.
Maybe he thought he didn't want Rigg to be part of the crowd backlash, as it would not have been helpful to his development.
It's only been two games since then, and in both games RLB has decided to play Hume to supplement what had become a leaky defence, with 9 conceded in two games. The formation changed and there was no room to play Rigg. If he'd come on against Wolves or Man Utd it would have required a change of formation, and RLB wouldn't want to do that with both games in the balance, especially the way the midfield played so well against United.