Maybe Mikey Garcia can step in? He's been working out and training and he's already gone 12 and beaten Lipinets... PBC/Haymon... Easy to make.
My guess is he was hoping it'd heal in time and it just didn't happen. It'd be highly irresponsible to be doing anything physical enough to lead to a back injury this close to fight time. I hope his team was honorable enough to disclose there was a chance he wouldn't be ready so a backup plan could be put in place.