Building depth is done when you draft well year after year, which is something the Bills haven't done in a long time. But if you're not drafting well in this era, you can somewhat off-set it with value signings in UFA A good example is a guy like Alan Branch who is playing this season on a reasonable contract.
When you don't draft well, particularly in the middle rounds, and aren't willing to make up for that with value signings, it's hard to maintain the sort of depth required to offset injuries. I don't expect Buffalo to spend all their cap dollars every season in a pursuit to establish depth, but it's ridiculous to believe them to not have injuries during the season. If Gilmore goes down, Rogers is still on the field. If Stevie goes down, it means an unproven type gets immediate PT. That's not what I expect from a franchise which tells us everything is supposedly new. It's their M.O. for going on years.