I don't know the details surrounding Rod Marinelli leaving the Chicago Bears and, to the best of my knowledge, those details have not been reported elsewhere. Marinelli could certainly have remained the defensive coordinator and led one of the top ten units in the league next year. Did he not want to work for Marc Trestman? Did he resent Phil Emery for firing his boss and buddy Lovie Smith? Did the promotion of Joe DeCamillis to the utterly meaningless role of assistant head coach rub him the wrong way? Was the lure of being on Monte Kiffin's final defensive staff in Dallas too strong? Any of these answers may be correct. Marinelli's decision is not.
Now Halas Hall's first true General Manager/Head Coach team - Phil Emery & Marc Trestman - must decide what coach will be charged with earning the trust of an installed veteran locker room while developing new talent on a consistent basis. It won't be easy for a new coach to step in front of Tillman, Briggs, Urlacher, Peppers...etc. and demand the love and attention the former defensive coaches did. Perhaps no one can.
But the new defensive coordinator might be looked at as more of a caretaker position than anything else.
Marc Trestman has been hired to re-imagine the entirety of the offense: new protection schemes, new passing game, new attack. Trestman's role is essentially to usher the Bears into the modern era of NFL football. The new defensive coordinator will have a diametrically different charge: don't fuck up a good thing.
Will it be a promoted John Hoke? Mel Tucker? Raheem Morris? Mike Singletary? We will likely find out by the end of the day what Trestman and Emery decide. One thing we know based on the track record of these two men? The decision will be handled in a thorough, meticulous manner.