When news first broke that Rex Ryan had agreed to become the next head coach of the Buffalo Bills, team sources leaking the info to media were quick to point out that incumbent defensive coordinator Jim Schwartz remained on staff, and could stay to coach under Ryan despite fundamentally different defensive philosophies.

That, however, does not appear to be the case. On Monday evening, several reports surfaced that Schwartz will leave Buffalo, and that Ryan will install his New York Jets defensive coordinator, Dennis Thurman, as his coordinator with the Bills.

Seems unlikely DC Jim Schwartz will remain w #Bills, despite Rex Ryan offering him chance to stay. Schwartz probably gone by tomorrow. — Vic Carucci (@viccarucci) January 13, 2015

As others have noted, Jim Schwartz will not return as Bills DC. As 1 source explained, differing defensive philosophies biggest reason why. — Adam Caplan (@caplannfl) January 13, 2015

Bills wanted to keep Schwartz but it wasn't going to work. Rex has Dennis Thurman, who was his DC the last 2 seasons. Bills D will be fine. — Adam Caplan (@caplannfl) January 13, 2015

It's an ironic twist of fate for a Bills defense that will now have a fifth defensive coordinator in as many years in 2015, because Ryan's defense is what the team ran a year prior to Schwartz's arrival in Buffalo, when Ryan disciple Mike Pettine was the Bills' defensive coordinator.

Stylistically, the Ryan and Schwartz defenses could not be much more different. Ryan runs a base 4-3 Under, which allows for formational disguise and requires positional versatility in his front seven; Schwartz runs a far more traditional base 4-3, and relies very little on disguise. Ryan is as aggressive a blitzer as you'll find in the NFL; Schwartz preferred to rely on a four-man rush. Ryan's coverage preferences (Cover 1 and 0, man-based) go hand-in-hand with his rush preferences, as do Schwartz's (Cover 2 and 3, man under, and much more zone). Clashing would have been an inevitability had Schwartz chosen to stay.

Instead, Ryan will re-install the defense the Bills ran in 2013 - his defense - and call the plays, while Thurman, who replaced Pettine as the Jets' defensive coordinator for the past two seasons, will assume the same role in Buffalo. The Bills improved dramatically from where they were in 2012 under Dave Wannstedt in Pettine's one year on the job, then made a seamless transition (and improved) in Schwartz's system. The expectation will be that a maturing young core of Bills defenders will make another seamless transition back, with only a few minor personnel tweaks to make.

To run a base 4-3 Under, the Bills will need a third defensive lineman to line up next to their Pro Bowl defensive tackles; the team can do better than Corbin Bryant (an exclusive rights free agent) and Stefan Charles. The strong-side linebacker position will also revert back to a hybrid position, with Manny Lawson poised to see an uptick in playing time from where he stood under Schwartz. On obvious passing downs, the Bills will also use far more dime than they did under Schwartz, with an emphasis on safety play - which could make retaining pending free agent Da'Norris Searcy a higher priority this offseason.

Again: with Ryan at the helm, and with a wealth of talent at all three levels of the defense, the Bills are going to be a very good defense in 2015, and that was going to be true whether they were running the Ryan defense or the Schwartz system. But if you're interested in the schematic tweaks, anyway, here are a few of our previous items on the Pettine-Schwartz transition to pique your interest.