Everyone seems to focused on WHO the defensive coaches are. One problem may simply be there aren't enough of them.
The staff has 7 offensive coaches (or offensive minded coaches) - Ryan Day, Tony Alford, Kevin Wilson, Brian Hartline, Corey Dennis, Greg Studrawa, Parker Fleming
They only have 4 on the defensive side of things - Larry Johnson, Kerry Coombs, Matt Barnes and Al Washington
Regardless of whether any of the 4 defensive coaches are replaced, it seems Day needs to replace at least one offensive guy with a defensive coach to help out on that side of the ball. With Coombs in the box, that leaves 3 defensive coaches on the field and one of them has to call the defense. They are simply understaffed on that side of the ball and its due to the composition of the staff.