Here's why I said linebackers are the key unit.

2012 stats (linebackers only) - Starters were Porter, Stewart and Jenkins for all intents and purposes. Leading tackler was Damontre Moore (also led TFL and sacks)

279 tackles (starters had 225)
20.0 tackles for loss (starters had 14.5)
10.5 sacks (starters had 8.0)
14 quarterback hurries (starters had 12)
3 interceptions/1 touchdown (starters had 2 INT and 1 TD)
3 forced fumbles (starters had 3 forced fumbles)
1 fumble recovery (starters had 1 fumble recovery)

2013 stats (linebackers only) - Starters were Jenkins, Claiborne, and Baggs for all intents and purposes. Leading tackler was Steven Jenkins, TFL Darian Claiborne, Sack tied between Gavin Stansbury and Shaan Washington.

328 tackles (starters had 215)
29.0 tackles for loss (starters had 16.0)
7.0 sacks (starters had 2.5)
5 quarterback hurries (starters had 3)
5 interceptions (starters had 1)
5 forced fumbles (starters had 4)
2 fumble recoveries (starters had 1)

2014 stats (linebackers only) - Starters for all intents and purposes were Bass, Alaka, and Walker. Leading tackler was Howard Matthews, Myles Garrett led TFL and sacks.

262 tackles (starters had 125)
19.0 TFL (starters had 5.0)
7.5 sacks (starters had 0.5)
14 quarterback hurries (starters had 4)
2 forced fumbles (starters had 4)
1 fumble recovery (starters had 0)

Justin Bass had a combined 2 tackles for loss and 0 sacks in 3 years under Mark Snyder, yet somehow he started every game last season