Dorito was a heck of a LB back in his day. But he was known for running headfirst into brick walls, not solving quadratic equations, or even remembering where he put his keys.
Diaz actually understands his D, and isn't looking in a panic for formula book answers. When your DC is flailingly scrambling to find printed answers to what he sees on the field, the one thing you know is he doesn't really know what he's doing. Doesn't make him dumb for sure -- the other possibility is his defense is too complex and he doesn't have a good feel for it. But either way, he's not serving as an effective DC.