I think the D will be fine with time. I'm not sure why everyone is panicking, this D has had three bad games, one to a damn good team in the Cowboys, one against the Bucs where they were on the field for almost 40 minutes of the game, and one against the Falcons.
Of all those games, I'd say the one against the Falcons should be taken on its own merit, that our D had a terrible game.
Things to correct the problem: insert Cole at RE, cut Pettaway and sign Malone from PS (or another DT in his place).
I think so much of the D's lack of consistency can be attributed to injuries. Bigby is a big loss, he was a great player against the run but also had an attitude that our D seems to lack.
I still maintain that when Harrell comes back, our run D will be fortified. Until then we need to find a temporary band-aid that covers the problem. 8 in the box (the 8th player being Bigby) would be my solution.
I also think Barnett has a more serious injury than is being let on. There is no way he should have regressed so much in a single off-season. He's been a secure tackler in his career in the NFL, to see him miss so much IMO points to his shoulder injury giving him more trouble than people realize.
Hawk's injury killed us. He seemed to be in slow motion, especially on the first TD where he was covering the TE.
I'd give Poppinga snaps at DE, and put Chilliar in his place. Incorporate some blitzes with Popp at DE that gives him the ability to drop back in coverage. Incorporate some new nuances into the D.
I still think it's too early to consider firing the DC, or making any dramatic changes.
As for O-line: I'm going to say let Chad and Taush work things out but keep them on a short leash. These two are our leaders on O, to bench either one would be unwise at this moment IMO. Clifton dominated Allen in the MNF opener, so I don't think he's lost "it".
Otherwise, just work on getting healthy as a team. I think injuries are being minimized in the role they have played on this team not playing to its potential.
The NFL: Where Greg Jennings Happens.