I don't see Richt ever replacing Manny with a big name coach, if we ever get another DC during his tenure it will probably be someone he elevates from within & I'm guessing it'll be a guy like Jess Simpson.
With that said, there are some pretty good DC's that don't get enough love in the media. And for the Super-fan sycophants NO I'm not saying all these coaches are better than Diaz, so relax & put the grenade down.
Mike Tressel at Mich St is one of the best in the game in building great Run Defenses year in & year out, despite the limitations of their program.
Danny Gonzales at ASU is a good up & comer that has already turned that Defense around, but was also a very good DC at SD State, they had 49 INT's in a 2 year time span in 2015 & 2016.
Lance Anderson at Stanford is another one who rarely gets the credit he deserves, but has built nothing but great Defenses at Stanford. Bob Shoop is good DC with a solid resume with stints at Penn St & Tenn.
Look at what Justin Wilcox is doing at Cal, he's a young guy that you can see even in his first HC job that he's going to be a monster once he gets to better program with more resources.
But then you have to remember, there are good young coaches all over college football & not every single one of them will always be on great teams. Even Saban snatched a guy like Pete Golding who was coaching at Southern Miss & UTSA before he was hired at Bama to be a Co DC.
Not every coach starts off immediately at P5 schools & hits the ground running, there's guys who are coaching at lesser schools who will eventually get a big time job.
I also think guys like Brian Jean-Mary, Jimmy Lake & Kevin Patrick will be great down the line.