It's been obvious since Week 2. The non Tyler Huntley qbs they have faced since the Seattle game are throwing 272 yards per game, 6 touchdowns, 2 interceptions on 69% accuracy. They can't stop the run, and can't stop the pass, constantly giving up big plays over the middle of the field.
The only reason to give Mayo the job was that the defense even with the injuries was a strength of the team last year and you want to try and have continuation on that side of the ball. The whole coaching staff deserve to be fired when no parts of the team are a strength this year.