The 49ers recently hired former Jaguars linebacker coach Robert Saleh to be the new defensive coordinator under head coach Kyle Shanahan.
The hire signals a shift from the 3-4 to 4-3 base defense. Prior to the becoming the Jaguars linebackers coach under former head coach Gus Bradley, Saleh worked as the quality control coach under Bradley for the Seattle Seahawks.
It makes sense then, the new Niners defense will be built in the mold of the Seahawks defense that was crucial to their 2013 Super Bowl victory.
The 49ers will reportedly be running a base 4-3 “under” defense (1-gap), as well the 4-3 “over,” both with 3-4 (2-gap) elements in it.