Ash has it right. The LBs -- particularly the MLB -- are a priority in his defense because he bases his defense out of a cover-2/cover-4 shell. While the safeties don't play particularly deep in his scheme, they have their own run gaps to worry about and can't provide much run support.
The Philadelphia Eagles used to run a wide-9, cover-4 system similar to what Ash runs (ironically, they may be returning to this style under new defensive coordinator, Jim Schwartz).
Here is one of the best articles I've read explaining the importance of having a solid/instinctive MLB in this type of system.
Overall, I get the sense that the MLB is the one position in his defense that will
require a "plus" player with the right blend of size, technique, & instincts.
Slightly OT, but here's another fantastic article about type of system Ash is going to install. Although it's for Michigan State, the same rules probably apply.
TL;DR - don't worry too much about Rene. The key to Ash's style of defense is the MLB.