Dallas fan here. (Yeah, I know.)
Was excited about Ryan initially. But don't get too worried about him in St. Louis. Totally overrated and gets by on his name IMO. That's not to say he's a terrible DC. He did OK with the ridiculous amount of injuries Dallas had on D this year.
But if St. Louis expects him to bring an aggressive, turnover generating defense, they'll be disappointed. He is incredibly conservative and will drive them nuts with his 3-man fronts and dropping everyone into coverage 99% of the time. Hardly any pressure on the QB.
His defense is definition of "bend but don't break." Which leads to the opposing teams being able to sustain long drives that eat up the clock.