A Look at the San Francisco 49ers New Defensive Scheme

The NFL is a copycat league. The Seattle Seahawks went to back-to-back Super Bowls on the strength of their defense. Robert Saleh, the new San Francisco 49ers defensive coordinator, was on the coaching staff of that defense. He will now look to recreate Seattle’s 4-3 defense in San Fran. This will require a scheme change to a 4-3 style of play, but the 49ers have the players to execute Saleh’s plan. Looking at how the 49ers will line up next year will help to fully understand the 4-3 under.

A Look at the San Francisco 49ers New Defensive Scheme

Defensive Line

The 49ers currently possess highly talented defensive lineman. DeForest Buckner, Arik Armstead, and Solomon Thomas are versatile players, and could play all the positions along the line. However, how they line up along the line will be the interesting part. The 4-3 under defensive line will essentially be made up of positions called a Leo, a three-technique, a one-technique, and a four-technique.

The Leo is the team’s best pass rusher, and is the most important piece of the defensive line. This player lines up on the outside shoulder of the weak side tackle. The Leo’s sole job is to wreak havoc on the quarterback. Although he is better suited to play inside, Armstead has been linked to the position the most. Aaron Lynch profiles as the best fit for the Leo, since he is the team’s best pass rusher. Due to off the field issues, however, Lynch may not even make the team.

The three-technique is an interior defensive tackle that lines up on the outside shoulder of the weak side guard. This position is usually a bigger bodied defensive lineman capable of stuffing the run, but also is a competent pass rusher. Buckner will most likely play here, with Quinton Dial getting some snaps as well.

The one-technique is most commonly referred to as the nose tackle. The nose tackle lines up on the strong side shoulder of the center. This position requires a big defensive tackle, responsible for stopping the run and stuffing the gaps. Earl Mitchell will be the 49ers nose tackle, with D.J. Jones developing as his backup.

The 4-technique, or the big end, lines up just inside of the strong side tackle. Michael Bennett plays this position primarily for the Seattle Seahawks. Since Solomon Thomas is often compared to Bennett, it makes sense that Thomas will play the four-technique. Both Armstead and Buckner fit the position as well, but Thomas is better suited for the four-technique.

Another key point to note is how the line will look in a nickel set. If the 49ers truly want to mirror Seattle, Mitchell will be subbed out, and Thomas will kick inside to the nose.  Armstead would then slide over to the four-technique that Thomas vacated. This will allow for a more pure pass rusher to take up the Leo spot. Aaron Lynch should be the nickel Leo, though Pita Taumoepenu or Eli Harold will vie for his spot should he get complacent.


Gone are the days when the 49ers linebacking corp was the envy of the league. A once feared group has seen the departures of Patrick Willis, Chris Borland, and Aldon SmithMalcolm Smith and rookie Reuben Foster will join mainstays NaVorro Bowman and Ahmad Brooks in hopes of restoring the linebacker group to prominence. In the 4-3 under defense, there are three linebackers with vastly different responsibilities.  The three positions are the Sam, the Mike, and the Will.

The Sam linebacker is the strong side linebacker. In the 4-3 under, the Sam linebacker will line up on the defensive line.  This will give the look that the 49ers have five defensive linemen. However, the Sam has more responsibilities. On running downs, the Sam will try to set the edge to contain outside running plays. On passing downs, the Sam will be asked to rush the passer, cover the tight end, or cover the running back. Look for Brooks to get most of the snaps as a pass rushing/run stopping linebacker, with Smith getting some snaps as the coverage linebacker. In a nickel set, the Sam will come off the field in place of the slot corner.

The Mike linebacker, or middle linebacker, is the captain of the defense. This player is responsible for calling the plays and aligning the defense. It requires a player who can run sideline to sideline, shoot gaps to stop the run, and is a sure tackler. The most obvious fit for this position is Bowman. However, Foster also profiles as a Mike. Foster would replace Bowman if he is injured or needs a break.

The Will linebacker, or weak side linebacker, is usually a linebacker that is great in coverage. In the 4-3 under, the Will closely resembles the Mike, and must be a good run stopper as well. Malcolm Smith is the prototypical Will linebacker. His playing time in Seattle make him a natural fit at the position. Foster can also play the position, and will try to beat him out in training camp.

Defensive Backs

The flashiest position of the 4-3 under is the secondary. However, it may be the hardest to recreate. The secondary usually plays cover-3, meaning two deep corners, and a deep safety in the middle of the field. The key characteristics that make this defense go are a ball-hawking free safety, a hard hitting strong safety, and physical, intelligent corner backs.

The most important position not only in the secondary, but the entire defense, is the free safety. It also is the most difficult to mirror. Jacksonville and Atlanta have also tried to implement this style of defense. They haven’t had much success due to never finding the perfect free safety. The 49ers have one ideal candidate for the job: Jimmie Ward. Ward played safety in college, but has played primarily corner in his short career. He profiles as a rangy safety with great instincts, capable of covering the middle of the field. If Ward gets injured or struggles, there isn’t a safety on the roster with the skills necessary to make this defense go.

The 49ers have two intriguing options at strong safety. Eric Reid offers veteran leadership, and has experience at both safety positions. Jaquiski Tartt is a physical specimen with all the tools to be the next Kam Chancellor. In this scheme, the strong safety will play up towards the line. This will allow him to be a force in the run game. Reid will most likely begin the season as the starter, but Tartt could very well finish as the starter.

The last piece to the secondary is the corner backs. Ward’s move to safety and Tramaine Brock‘s cutting hurt the depth at corner. Rashard Robinson showed flashes as a rookie, and will be asked to be the number one corner. The number two spot is up for grabs. Third round pick Ahkello Witherspoon fits the mold of what Saleh is looking for in his corner backs. Witherspoon possesses great speed, length, and intelligence. Dontae Johnson and Keith Reaser will share time at corner if Witherspoon is unready to start. K’Waun Williams and Will Redmond will compete for nickel spot.


It is unrealistic to assume the 49ers defense will transform into one of the best in the league in just one year. Building a defensive juggernaut takes time. However, the 49ers should be vastly improved on that side of the ball in 2017. The 49ers have the building blocks and the coaches to take the first steps towards prominence. For the first time since Vic Fangio’s departure, the 49ers have a defensive coordinator with a plan. The new defensive scheme will allow for the three first round picks along the defensive line to shine. It will also play to the strengths of all the linebackers and members of the secondary. With any luck, Saleh can build a defense that will carry the 49ers to their sixth Lombardi trophy.