And more specifically, Rex is related to the extent that he didn't provide the perfect conditions for Mario to succeed. But that wasn't his !@#$ing job. But I've already stated very clearly, that Rex erred in attempting this by merging schemes.
On the other hand, it's Mario's !@#$ing job to do what he's being asked to do and be as successful doing it as he can be. He clearly didn't do that. And he's clearly not doing it now. He should have been benched.
So if the concession here is: Rex didn't design a defense to feature Mario Williams, then sure. If that's the case, Rex played a key part in Mario's decline. What that has to do with his decline in Houston and Miami, I'm not sure.
But to the extent that Rex designed a defense that Mario opted out of, no. Rex played no part in his decline, as evident by the players who are succeeding this year and have--under Rex--in the past, to say nothing of Mario's own personal record in similar matters, with different coaches, on different teams.