The 49ers enter the game against the Buffalo Bills with a defense known for shutting down the run. They haven't allowed a rushing touchdown in 11 straight home games and haven't allowed a 100 yard rusher in their last 21 contests in San Francisco. But historically they've been threatened much more by players who get outside of the teeth of their defense...guys like C.J. Spiller.
They've also had issues slowing down screen and swing passes where the secondary is responsible for coming up and making a quick tackle to prevent a big gain. The 49ers secondary are known for their big hits, but often times they go for the crushing blow rather than wrap-up for the sure tackle.
A running back like Spiller presents all of these challenges to the 49ers as he and the Bills come to San Francisco to invade Candlestick Park. The outside linebackers (when in base 3-4 defense) and the defensive ends (same guys, only in Nickel package) will have to be honest in setting the edge, resisting the urge to crash the middle in an attempt to make a play.
Thus far Aldon Smith has shown pretty good speed getting to the edge against RBs and QBs alike. Having guys like Patrick Willis and NaVorro Bowman, who are excellent sideline-to-sideline players, also helps the 49ers chances of swarming to make tackles on the edge.
Still it will be up to the secondary to make solid tackles, contain players and force them back inside to their help, and rallying together to make gang-tackles.
Chan Gailey likes to line-up in shotgun formation a fair amount of the time. This typically involves three or more wide receivers, which the 49ers tend to counteract with their nickel package. This puts pressure on the secondary to make plays as there are five of them on the field with the nickel DB on the strong side often being responsible for plays on the edge. This player has been Chris Culliver thus-far, who'll need to have a strong day tackling.
Ultimately I don't expect to see Spiller rack up a ton of yards on Sunday, but we might see a few good plays to the outside, which has been a sore spot for the 49ers.