Jim Bates' defensive system that worked so well in his previous stops never took hold in Denver, so he resigned from the Broncos on Tuesday.
The Broncos finished 19th in overall defense and 30th against the run, drops from a year before when coordinator Larry Coyer was fired.
Rather than accept a reassignment to the staff, Bates decided to walk away with two years remaining on his contract.
"I had the opportunity to stay," Bates said in a written statement. "(Coach Mike Shanahan) has been very fair with me, and the final decision, for the Broncos and for me, was to step aside."
Bates, whose title was assistant coach-defense, had success with his system with the Green Bay Packers and Miami Dolphins, but the Broncos were unable to master it.
A few weeks into the season, the Broncos were allowing nearly 200 yards a game on the ground and decided to ditch the hallmarks of Bates' ballyhooed system, replacing their big, beefy interior linemen with smaller, quicker tackles who could pressure the passer and putting safety John Lynch in the box as an eighth defender against the run.
Things improved but the progress came haltingly and the Broncos (7-9) ended up allowing 409 points, fifth-most in franchise history, while posting just the second losing record in Shanahan's 13-year tenure.
Bates said he was "very saddened that things did not work out."
Team spokesman Patrick Smyth said the Broncos would have no comment beyond Bates' statement. Shanahan has scheduled his season-ending news conference for Thursday, when he's expected to announce that Bob Slowik will assume Bates' duties.
Slowik was promoted to defensive coordinator last year after Bates was hired as assistant head coach in charge of the defense. Slowik also was in charge of the secondary.
Slowik served as defensive boss once before, for Mike Sherman in Green Bay in 2004. That didn't work out well and Slowik parted ways with his good friend and joined the Broncos the following season.
Some players suggested Bates' system didn't take because there was so many new faces on the line and others blamed too much holdover from Coyer's philosophies.
"I just don't think we had any consistency or any confidence at any point in ourselves as players or in the scheme that we were running," defensive back Domonique Foxworth said last week. "I think it's important that we build that into the next season, that we find something that we're good at and we stick with it from start to finish."
Defensive lineman Kenny Peterson, who worked for Bates in Green Bay and benefited from the change in personnel and philosophy as the season progressed, said: "It was unfortunate it didn't work here because I like his system. He has a good system, it's proven, it's tested."
Linebacker Nate Webster said too much youth put the Broncos in a pickle, but Foxworth suggested the problems stemmed from the scheme actually being a hybrid.
"It wasn't 100 percent commitment, I don't think, from Day 1 to the new scheme," he said