Nolan's career as DC has been very up and down. Seems he's done well when he's had talent (Bal 2002-2004 - had #1 defense by DVOA in 2004, with top5 finishes all 3 years - with otherworldy Ravens talent), NYJ 2000, MIA 2010-2011), but has been awful with mediocre to bad talent (Wash 1997-99, ATL 2013-2014, SF Head Coach).
So, as many writers/analysts have pointed out, talent means more than scheme for defensive success. I think most on here would agree that Dallas is lacking talent at several key defensive positions; DT, LB, S. Good luck to whoever's the DC. At least Marinelli's influence will be gone on draft day.