Rex's Defense wasn't the problem when he was losing for the Jets. He lost because of the offense. His fault, yes, but his defense wasn't some tire fire except for the last season.
He makes bad coaching decisions. He doesn't know how to handle an offense. He seems clueless how to instill discipline in a team without his players losing their edge.
There's many things to criticize Ryan for, but his defensive scheme is not one of them.