49ers will be running a 4-3 defense in 2017 under new defensive coordinator Robert Saleh. Saleh was a defensive quality control coach with Seattle under Pete Caroll from 2011-2013. Saleh followed then-Seattle defensive coordinator Gus Bradley to Jacksonville, where Bradley became the new head coach. Saleh stayed with Bradley in Jacksonville from 2014-2016 as a linebackers coach.
Now, he finds himself in San Francisco running the 49ers defense under new head coach Kyle Shanahan. Saleh will be bringing the Seattle 4-3 defense with him to the Bay. The big question is; who will be playing where in this new defense? As the 49ers are changing from a 3-4, which they have played in for years going back to pre-Harbaugh era, to the 4-3 under Saleh.
First, lets look at what the base 4-3 defense could look like next season. This still is from Week 2, 2016 in Jacksonville.
Week 2, 2016 Chargers at Jaguars
We have 4 defensive linemen, 3 linebackers weak, middle, and strong. 2 outside corners, and 2 safeties. One closer to the box and one covering center field. Big difference with this defense compared to what the 49ers have been running the past few seasons is the single high safety look, which is the what we will see on most snaps.
On the defensive line we have the 0 tech (over the guard) which will most likely be played by newly acquired defensive linemen Earl Mitchel. Then the weak side (away from the TE) 5 tech, who could be played by either 2015 1st round pick Arik Armstead, or 2016 1st round pick Deforest Buckner. Same could be said about the strong side 3 tech – either could play the position. Now the player lined up over the TE, which is refereed to as the Leo, could be played by Aaron Lynch. Lynch played OLB in the 3-4 defense last season and this transition will be an important one if the defense is going to be successful.
When it comes to the linebacker group, assuming Bowman is healthy to start the season, he would be the middle-linebacker. Malcolm Smith signed a 5-year deal with the 49ers under the intention of playing the weak-side linebacker. Question is; who will be playing the other linebacker position? Right now it looks like that would be Ahmad Brooks.
Free safety is probably the most important player in the secondary in this defense. Jimmy Ward is the only guy on the roster that seems like he could play the position at a high level. Reid and Tart will be more box safety kind of players. The 49ers could address the free safety position in the upcoming draft early as players like Malik Hooker and Jamal Adams seem like good fits.
The transition from 3-4 to 4-3 can sometimes be a difficult one, but this new 4-3 that the team will be running is very versatile and puts players in positions to succeed.