Depth is one of many issues. Bo was quoted at the beginning of the year that there was more "distance between the 1's and 2's than he would like in some positions".....
Bo has historically IMO, been reluctant to sub guys throughout the lineup in an attempt to develop meaningful depth (coaching issue). I think this is evidenced in the lack of depth at DT, Safety, LB etc.... (recruiting) as well as the QB position.
We have "depth" in that we have back ups, but the drop off is significant IMO. Mix this in with Bo's NFL mentality of you "lose" the position to injury or graduation and it has come home to roost. Off thread, but this is shown in his reluctance to "sit" guys for a series or 2 who are blowing it on the field.....Year 7 and we have no more quality depth in any position save WR and RB IMO. Not to mention, IMO, our 3 best OL were 2nd string and sitting on the sidelines most of the year...
Depth is developed from recruitment, playing time and player development........the one commonality? Coaching.