The defensive line's success is predicated on high energy, 100% effort every play. Realistically, this cannot be achieved by a single player at each position. Therefore substitution is critical. You must have backups that play almost to the same level as the starters. Really, there are no backups, just a group of 6 to 8 part-time starters. It worked well in the early 90's when we had the bodies.
We just didn't have the bodies today. We were missing two potential high contributors in Hardy and Gregory. Then we lose Mincey. They got tired quick.
McClain in the middle would have stopped a lot of those gashing runs by the Falcons.
Then on offense, we lose an All Pro WR and a Pro Bowl team leader QB. Without the field stretching Bryant, the offense was exposed.
A QB that lacks experience in the system is going to be limited and the play calling is going to be limited. Combine that with the loss of Bryant and the big play potential shrivels. You also have a rookie on the offensive line with his very first NFL start.
Injuries were a very large contributor in this loss.