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49ers defense will take on different look under Robert Saleh

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What does Saleh hiring as DC mean for 49ers? (1:40)

Bill Polian says new 49ers defensive coordinator Robert Saleh has his work cut out for him with the team's current defensive personnel. (1:40)

SANTA CLARA, Calif. -- The San Francisco 49ers are soon expected to officially announce that Robert Saleh, the former Jacksonville Jaguars linebackers coach, will serve as defensive coordinator under coach Kyle Shanahan.

Considering Saleh's background working for Pete Carroll with the Seattle Seahawks and Carroll disciple Gus Bradley in Jacksonville, it's expected that the Niners will be making some changes to their defensive scheme. At a basic level, the Niners will be changing from a 3-4 front to a 4-3, leaving some to wonder if some of the premium draft picks the Niners have used on defense in recent years will no longer be a fit. But here's the thing: It doesn't really matter.

First and foremost, Carroll's Seattle defensive scheme is far from a "traditional" 4-3 scheme. Carroll has described his defense as a 4-3 front with 3-4 personnel. It's a method he told ESPN's Sheil Kapadia he honed under the guidance of Monte Kifin at the University of Arkansas in 1977 as a graduate assistant.

"That really came out of all the way back to Fayetteville," Carroll said. "It was a one-gap principle defense way back then. Monte and I, when we were together in Minnesota with (former Vikings defensive coordinator) Floyd Peters, we were able to continue to build on and NFL-ize our scheme. And so, it’s been a way to get guys to attack and play aggressive so that you can get good pass rush and still fit on the run. That’s where the secondary fits and stuff like that really comes into play."

Beyond the fact that the scheme doesn't necessarily resemble a typical 4-3 or other versions of it, such as the wide-nine scheme, the other piece of the puzzle is the fact that because offenses are so regularly using three or more receivers, teams don't use base defenses as much as they once did.

According to ESPN Stats & Information, the Niners were in base defense (defined here as four or fewer defensive backs), for 687 snaps in 2016, the most in the NFL. Still, that was only 62.3 percent of their total defensive snaps, meaning they had five or more defensive backs on the field more than a third of the time.

For sake of comparison, Seattle and Jacksonville both ranked among the top eight in the NFL in terms of total snaps played in base defense, but came in at 57 percent and 62 percent, respectively, in terms of time spent in base defense. The 49ers, Seahawks and Jaguars were all in the top five among fewest snaps played in sub packages.

So what could the Niners' new base defense look like? The 4-3 "under" front is a gap-control scheme largely focused on stopping the run and allowing certain players to generate pressure on the quarterback. It also allows for variety in the terms of looks even if the principles remain the same. While Carroll's defenses have stuck to similar basics for most of his time in Seattle, he's also been flexible within that scheme to adapt to his talent.

For the 49ers, there will be some of that give and take as Saleh and his staff work to figure out where the pieces fit. But to hear general manager John Lynch and Shanahan describe what they want their defense to look like, the words "attack" and "aggressive" are common.

"Here’s what we know on defense: We want to be aggressive," Lynch said. "We want to be multiple. Kyle talked a lot about what gives him problems as an offensive coordinator, so I think those are things that we’re looking for."

For the most part, the 4-3 under asks each defensive lineman and linebacker to be responsible for one gap, which increases players' ability to play instinctively instead of overthinking. There is also some wiggle room for two-gapping, usually reserved for the left defensive end and/or the nose tackle.

In its basic alignment, the left defensive end (playing 5-technique) and nose tackle (playing a 1-technique) line up to the closed side (side with the tight end) of the formation with the SAM linebacker lined up over the tight end. The nose tackle and left defensive end are generally expected to be your best run-stoppers while the SAM linebacker must have the ability to set the edge against the run and drop in coverage consistently.

On the other side of the formation (the open side), you often will find the defense's two best pass-rushers, a 3-technique defensive tackle and what is known as the "Leo." The Leo lines up to the outside shoulder of the weakside offensive tackle, leaving the middle and weakside linebackers to handle the strongside 'B' gap and the weakside 'A' gap, respectively.

Coverages to pair with this front can vary depending on personnel, though the Seahawks have long been proponents of the Cover 3, largely because of the presence of free safety Earl Thomas. For Seattle, the perfect mix has included an athletic free safety paired with a physical strong safety who can play in the run box to complement long-limbed corners capable of playing press coverage.

So, how does the current Niners roster fit in with the projected change? That remains to be seen, but there are some reasonable matches that can be expected. Defensive lineman DeForest Buckner would be a logical fit as the 3-technique defensive tackle. Outside linebacker Aaron Lynch makes sense as an option for the "Leo" spot and NaVorro Bowman and Ray-Ray Armstrong have the characteristics to step in at middle and weakside linebacker jobs. The Niners also have corners such as Rashard Robinson and Tramaine Brock who could fit the mold outside. Upgrades at nose tackle and strongside linebacker, as well as more punch for the Leo spot, would appear to be the biggest needs in the front seven.

Still, there is sure to be a feeling out process for Saleh and the players on the roster in determining how all the pieces fit. No matter how the Niners line up from play to play, Shanahan has a bigger picture of what he wants to see from his defense.

"[I like] defenses that make you work for everything, that are extremely sound, that make you one-dimensional; they stop the run, make it a one-dimensional game, and they don’t give up anything for free," Shanahan said.