The Buffalo Bills will be moving to a "hybrid" defense under new defensive coordinator Mike Pettine - one that will operate under the premise of "attack first" out of multiple fronts. It's the type of defense that teams have to employ to keep up with evolving offenses these days. It also makes talking about what the team needs on that side of the ball somewhat confusing.
In order to potentially delineate what the Bills should be looking for defensively this off-season, we need to start from the beginning: determine the team's existing building blocks on defense (moving front to back), then fill in the missing pieces. It's very safe to assume that the Bills have three such players, and they all happen to be defensive linemen: Mario Williams, Kyle Williams and Marcell Dareus. Let's quickly review what each player can do best:
Mario Williams: He's best as a power end, playing either the six or the seven-technique on the outside of the offensive tackle. In a traditional 4-3 defense, this would mean he'd be playing on the strong side; in a hybrid defense, that would be far less commonplace. Williams can also drop into short zone coverage on occasion and play in space some, but has also said in the past that he doesn't prefer doing so; he could theoretically slide inside to play some three- or four-technique in pass rushing situations, as well.
Kyle Williams: He's at his best playing a three-technique, meaning that he's lined up on the outside shoulder of an offensive guard (almost always on the weak side of the formation). From there, he's assigned one gap responsibility against the run, and he's supposed to get into the backfield. Williams has also played a lot of one-technique in his day, but has trouble working through double-teams because he's not the biggest guy in the world. Kyle is pretty much limited to these two roles, and the three-technique is preferred.
Marcell Dareus: A much more versatile player than either of the first two names, Dareus is at his best right now as a three-technique, but has the upside to be a very good one-technique player - though that would limit his opportunities to get into the backfield, as he'd be asked to take on more double-teams. He's unique in that he has sufficient athletic measurements to moonlight in two-gap looks as a zero-technique (where his bulk comes in handy) or a five-technique where his arm length is enough to handle massive offensive tackles). Dareus has only scratched the surface of his potential, and if he does, he'll be the true centerpiece of the front seven in a hybrid defense.
Those are the front-seven players that will be guaranteed playing time for Pettine for the foreseeable future. More every-down capable players are needed, and in these areas in particular:
Play-Making Linebackers (Yes, Plural)
Well, obviously. Buffalo released its only three-down capable linebacker earlier this week in Nick Barnett, so now they need to replace him with another linebacker that can play three downs. Ideally they'd have two, much like San Francisco's embarrassment of riches in Patrick Willis and NaVorro Bowman. Three-down linebackers, remember, can play in space, can drop, and can play the run. They are not particularly easy to find, particularly for a hybrid defense, where size is a bigger factor than, say, in a strict Tampa 2 defense.
Right now, the Bills have one linebacker that could potentially one day be a three-down player - Nigel Bradham. The other projected starter would be Kelvin Sheppard. Clearly, this is Buffalo's biggest area of need defensively. Don't be surprised if the Bills use a very early pick (read: first or second) on a three-down capable linebacker. Four names to keep in mind are: Manti Te'o (Notre Dame), Kevin Minter (LSU), Alec Ogletree (Georgia) and perhaps even Arthur Brown (Kansas State) despite his smaller frame.
Note that this position could be inclusive of the one below - but ideally, it isn't.
SAM Linebacker, Pass Rushing Hybrid
In order to consistently pressure passers and hold up against the run on the strong side of the formation, hybrid defenses typically sacrifice some coverage ability for players that can play linebacker in base sets (i.e. drop into short zones and stack/shed against the run), but who can also put a hand in the dirt and rush the passer in nickel sets. The idea of a hybrid defense, remember, is to be able to use the same personnel at all times, rather than field a team of situational specialists.
If the season were to start tomorrow, the Bills would be picking between Kyle Moore, Mark Anderson and Chris Kelsay (and hey, maybe even Arthur Moats!) for that role. Anderson is not the typical power-oriented player out there (nor is Moats), while Moore has very limited experience dropping into coverage and playing in space. (Kelsay has that experience, but we all know how that has turned out.) Theoretically, the Bills could get away with one of those guys in the short-term, but they need a player specifically for that position at some point.
Baltimore filled this position for a number of years with two draft picks: Jarret Johnson (a 260-pound fourth-round draft pick in 2003) and Paul Kruger (a 270-pound second-round pick in 2009). That's Baltimore's prototype, however; other teams employ more athletic types (see: Von Miller). Either way, the idea is to find a good player that can reliably perform both duties - even with limitations - and not necessarily to plug a player in.
Kruger is a free agent, and the underrated Manny Lawson (also a free agent) capably plays that position in Cincinnati. This type of position is also why scouts are drooling over the upside of a player like Dion Jordan of Oregon (and he's not even close to the only prospect that could work out here). It should not be Buffalo's top priority, but this is a role they need to fill with a good player.
Buffalo is very young at cornerback, and really only has one player that we can reliably claim as a building block (that'd be Stephon Gilmore, of course - and he could have been covered in the "building blocks" section very easily, were we not focused on the front seven first). They have one other intriguing cover corner that's raw with upside (Ron Brooks), and a couple of intriguing "interior" cover guys (Aaron Williams and Da'Norris Searcy) that are harder to define because neither are (or should be, in the case of Williams) perimeter coverage defenders.
Pettine is a Rex Ryan disciple. Rex Ryan collects cornerbacks, largely because his hybrid defense sacrifices a bit of coverage in that hybrid linebacker role; couple that with an astonishing amount of overload blitzes (which Pettine will likely employ in Buffalo), and it's essential that a secondary is in place that can play good man coverage often. We can reasonably expect Buffalo to start collecting cornerbacks - and ideally, they'll be more athletic-minded (read: less concerned with size) in doing so. Natural coverage corners are an extremely valuable commodity in an increasingly hybrid NFL.
From there, the Bills can fill out their depth chart with prospects for the key roles and a few specialists (like, say, a deep stable of linebackers and a safety or two, plus a defensive lineman that can reliably play the zero- and one-techniques to keep Williams and Dareus freed up). From a volume standpoint, aside from cornerback, the Bills only really need three players (and they're all linebackers, with one being the aforementioned pass-rushing/SAM hybrid). There are other interesting players to work with not previously mentioned - Alex Carrington is the first name that comes to mind - but they're role players. Buffalo needs more than role players - they need two dynamic linebackers and a better hybrid pass rusher to have the prototype for what Pettine needs defensively.