For the past two seasons, San Francisco 49ers defensive coordinator Robert Saleh has tried implementing a 4-3 Under, Cover 3-style defense — a virtual clone of what he experienced when working as a defensive assistant with both the Seattle Seahawks and Jacksonville Jaguars before joining the Niners ranks in 2017.
In some regards, it worked. San Francisco’s previous attempts to retain a 3-4 defense weren’t particularly effective, especially after defensive end Justin Smith retired in 2015. Last year, the 49ers also ranked seventh best against the run, allowing a mere 4.1 yards per carry to opponents.