The defense's losses are more spread out than the offense's; hopefully that gives the new starters time to grow into their positions.
DT Brian Arnfelt
Arnfelt was Northwestern's best defensive tackle in 2012. While he wasn't a dominant presence in the middle, he was the only DT who somewhat consistently played on the offense's side of the line of scrimmage. Since none of the other DTs showed anything comparable to Arnfelt's best plays, he looks like the hardest senior to replace.
DE Quentin Williams
Williams had his best game in his final game, and he showed why he earned a starting spot all year, finishing second on the team in sacks and third in tackles for loss. He also showed the coverage skills that Northwestern needs from defensive ends, finishing the year with 3 passes broken up and an interception return touchdown. Still, defensive end is a deep position, and both Dean Lowry and Deonte Gibson showed good pass rushing skills. We will see how that works out with more playing time.
WLB David Nwabuisi
Nwabuisi finished second on the team in tackles, with the other two linebackers on either side of him; this is as things should be. He also recorded 8 tackles for loss, fourth on the team and second among linebackers, and he contributed 1 sack, 3 interceptions, and 4 pass breakups against the pass. While we didn't see much of the reserve linebackers in 2012, there are some veteran players who should be able to provide adequate production from the weakside linebacker position; with Damien Proby and Chi Chi Ariguzo returning Nwabuisi's replacement doesn't need to be a star to give Northwestern a solid linebacking corps.
Carpenter held down a starting spot at safety without providing many memorable plays. Indeed, 10 of his 66 tackles came in the Gator Bowl, and he only recorded 1 tackle for loss all year; this is the profile of a safety who spent most of his time in deep zones and whose front seven didn't let many runners through. Carpenter only recorded 1 interception on the season, though he did break up 6 passes. I hesitate to express confidence in Northwestern secondary play, but Carpenter's replacement only needs to avoid screwing up to be a serviceable player. I think this won't be a huge loss
CB Demetrius Dugar
Dugar got the start against Syracuse, and it didn't go well. While he played better the rest of the year, most notably coming off the bench against Indiana when Nick VanHoose was having a rare off day, he still wasn't quite where you would like a senior to be in coverage. Still, he managed to record an interception against Michigan and contributed a lot of snaps to the improved pass defense.
CB Quinn Evans
The Stanford transfer was the more reliable of the two senior corners, though he also had a blooper-reel play against Syracuse which allowed one of the 3 50+ yard pass plays Northwestern gave up in 2012. Evans didn't manage an interception, but he lead the cornerbacks in tackles (not entirely a good thing) and broke up 3 passes. Like Dugar, he played a lot of snaps for an improved pass defense, but after VanHoose returned from injury Evans found himself on the bench.
There are a few more defensive players leaving, but they got minimal playing time outside of special teams.
Tackles: 331 of 843 (34%)
I really have no idea what to think of this number. The more important point, I think, is that the second and fifth leading tacklers are gone, both of whom played positions which are expected to make lots of tackles. Northwestern needs to find a weakside linebacker who can soak up runners like Nwabuisi did and a safety who can limit long gains; if those holes are filled, the exact tackle distribution should be irrelevant.
Tackles for Loss: 24 of 69 (34.8%)
Quentin Williams led the charge among seniors, with 8.5 TFLs. That was third on the team; it matched the team lead from 2011, one aspect of the improvement from that disastrous season. Nwabuisi (8) and Arnfelt (6) followed in fourth and fifth, while Jared Carpenter chipped in with 1 and Quinn Evans contributed a half. This category highlights Arnfelt's contribution, as the next defensive tackle was Sean McEvilly with 1.5; at least one of the remaining tackles needs to show the ability to disrupt plays in the backfield.
Sacks: 8.5 of 28 (30.4%)
Quentin Williams was second on the team with 4.5 sacks (more than any defender recorded in 2011), but he still only managed half of Tyler Scott's sack total. Arnfelt was a standout again, tying Chi Chi Ariguzo with 3 sacks for third on the team. With Scott, Ariguzo, Drew Smith, Deonte Gibson, and Dean Lowry returning, replacing Williams rushing off the edge shouldn't be too difficult; the question is whether someone can replace Arnfelt's interior pressure on early downs.
Pass Breakups: 20 of 63 (31.7 %)
Carpenter led the seniors with 6, good for third on the team; the other 14 were spread around. David Nwabuisi did an impressive job of picking off the passes he got his hands on, recording only 4 pass breakups to go with his 3 interceptions.
Interceptions: 6 of 13 (46.2%)
Nwabuisi tied for the team lead with 3, while Dugar, Williams, and Carpenter each chipped in 1. This is both the category in which the seniors made the biggest impact and the most volatile due to the small number of interceptions and the vagaries of deflected passes. Nwabuisi looked better than I expected in the pass defense categories; this is a place where his presence might be sorely missed in 2013.
Instead of a starting lineup, I will give my thoughts on how rotations will work
Tyler Scott has a starting spot essentially secured; coming off the best individual sack season for a Northwestern player since Corey Wooton recorded 10 in 2008, he would have to perform extremely poorly in the offseason not to take the first snap against Cal. Either Dean Lowry or Deonte Gibson will start on the other side; this battle will probably focus on who plays better against the run. Mike Hankwitz likes to rotate 4 defensive ends, sometimes playing all 4 in passing situations, so this leaves another spot open. Ifeadi Odenigbo is an obvious possibility, particularly as a situational edge rusher, though he is currently listed at linebacker and may have a better build for that position. The other defensive end-sized bodies on the roster is sophomore Max Chapman and redshirt freshman Connor Mahoney, though Mahoney was recruited as a defensive tackle and will likely have added to the 260 pounds he is currently listed at. Incoming freshman Eric Joraskie could follow in Dean Lowry's footsteps and secure a spot in the rotation as a true freshman; I really have no idea who the fourth guy will be.
Sean McEvilly earned a starting spot as the season went on, and I expect him to retain that in the offseason. Will Hampton seems like the most likely tackle to fill the other starting slot, while Chance Carter should stay in the rotation. Redshirt freshman Greg Kuhar was a solid recruit who could push for playing time after a year in the weight room, while C.J. Robbins and Connor Mahoney are redshirt sophomores who haven't yet seen much action.
Finding a starting lineup here will depend as much on finding the right fit between players and positions as finding the three best players. Damien Proby will most likely remain in the middle, where his solid play helped clog interior run lanes. Chi Chi Ariguzo thrived on the strongside, where he finished second on the team in tackles for loss and third in tackles, sacks, and interceptions. Still, Northwestern has a few other players with potential at that position; Drew Smith picked up 2 sacks in minimal playing time, while it is the most likely landing spot for Ifeadi Odenigbo if he stays at linebacker. Ariguzo, then, could move over to the weakside to replace David Nwabuisi if the coaches want to move somebody else to the strongside. Collin Ellis has some experience as a starter from 2011, and he is the simplest choice to replace Nwabuisi, which would leave Ariguzo on the strongside; if I had to guess right now, I would expect Ariguzo-Proby-Ellis to be the starting 3.
Both bowl starters return, so I expect Daniel Jones and Nick VanHoose to be the starters again at the start of the 2013 season. Behind them, C.J. Bryant appears to be the next guy up; if VanHoose has injury issues again, we could see a lot of Bryant. After Bryant I know very little about the other options, so I won't even speculate about the other backup; hopefully we don't see much of him.
Ibraheim Campbell should be safe at one spot, while the other has three obvious contenders: senior Davion Fleming, junior Jimmy Hall, and sophomore Traveon Henry. Henry played mostly on special teams, but I thought he showed potential during his time on defense. Hall worked his way into the nickelback position at the end of the season, recording a critical pass breakup in the Gator Bowl that showed his potential in coverage. Fleming is the veteran pick, who hopefully knows the defense well enough to avoid the breakdowns that plagued 2011. Whichever player wins the position will likely be asked to be the primary deep middle defender when Northwestern puts a safety in the box. Whoever finishes second in this competition will probably play the nickel.