I don't know what kind of coach Marinelli is but I do know one thing, you can't give a chef ground meat and expect Chateaubriand on your plate.
There are two kinds of DC's in the game. One tries to tailor his D to the available talent, Marinelli, Phillips and Zimmer and the other Rob Ryan, runs his D regardless of the talent as he did in Dallas with disastrous results.
Marvin Lewis and Buddy Ryan got HC jobs based on their D's as DC's and were never able to even approach that standard again. Monte Kiffin never had a great D outside of TB with a loaded D roster. The best DC in the game doesn't have a system, it is a conglomerate of systems that he can fit to his talent in NE.
When he was the DC for Parcells with the Giants, he didn't create that D, the presence of Lawrence Taylor and what he could do did. Once he saw Taylor in action, be began to run it around him.
When the Cowboys moved to the 3/4 under Parcells, he was asked in a pc about the difference. He said the 3/4 is more dependent on the one on one matchups and the player winning at that point and very unforgiving if he doesn't. He never ran the 3/4 properly in Dallas because he didn't have the talent. Parcells ran more BDBD in Dallas that anywhere else he ever coached. He had to protect his D, it was seldom the 3/4 attack D he'd had at his other stops.
I think Marinelli tried to cover for the lack of talent and I think that diminished the effect even a good player could have because they were more worried about the other guy doing his job. "Just do your job" is what D is all about. And it can be a s simple as one guy.
Think about Roy OU Williams with Woody back there and without. Think about Ware without Ratliff's motor in the middle.
Landry's Flex D wasn't great until he got the great talent to execute it. The 4-6 doesn't work without those key players. What comprised the Steel Curtain, the DC or those players? Half are in the HOF. Is the Fearsome Foursome fearsome without Deacon Jones?
Marinelli, like this before him, had the challenge of playing for a follow the ball GM that still thinks it was triplets that got 3 rings.