A true defensive coordinator is the missing piece, it's kinda like when you have a QB rotation but you're not totally confident that either one is the best choice as the started... the co-defensive coordinator thing doesn't seem to help give the D an identity. Look at the Tennessee Titans, they've struggled for years on D then they bring in Greg Williams (after his bounty-gate suspension) and this guy know as a defensive genius has them playing better than they've looked for years. Teams really do seem to take on the personality of the main guy calling the shots from the DC spot.
Again I like Fickell and Withers but I'm not sure the D has a true identity and maybe it's hard to pick up on the personality to take after when the DC job is split. Maybe you take on a split personality? I don't know?... but I can see Urban making a change within the next year or so and bring in a bigger name with more credentials to try and solidify this D.
I know there's some young guys out there but the current talent level of the players isn't matching the results they expect...Cal scored more on us than they scored on much lesser teams with much lesser talent. Unless they really start to gel and become dominant this season, I think Urban will have only 1 true DC next season and it may not be Fickell or Withers (yet 1 or both will likely remain on the staff).