With the spotlight trained on Colin Kaepernick's ongoing development and Jim Harbaugh's prickly personality, it's easy to forget that San Francisco's success the past three years is largely due to its defense. Last season, analysts marveled at how this unit was talented enough to play a straight two-deep, man-to-man, four-rush scheme every snap with little substituting—a rarity in today's NFL.

But that aversion to subbing proved costly down the stretch. With All-Pro end Justin Smith battling through a torn triceps, the 49ers' fatigued front seven generated just seven sacks over its last five games, exposing a secondary that, when QBs were given time, was no longer great in man coverage. The Niners reached the Super Bowl only because Kaepernick's ascent coincided with the D's decline.

Kaepernick, who's as raw as he is talented, has had a predictably up-and-down encore season. Opponents have identified pressures and coverage rotations that exploit his flaws in the pocket. Receiver Michael Crabtree's return last month from a torn right Achilles has masked some of the QB's deficiencies, but not enough for this offense to drive another Super Bowl run.

Instead, the Niners must go back to leaning on their defense, which is more diverse after coordinator Vic Fangio used a wide array of personnel to compensate for midseason injuries. Young linemen Tony Jerod-Eddie and Demarcus Dobbs have played 691 snaps between them, creating depth behind a fresher Smith and the underrated Ray McDonald. Backup outside linebackers Dan Skuta and Corey Lemonier both excelled during Aldon Smith's six-week absence for substance-abuse rehab. (The long-armed, violent-handed Smith rounded back into form before the holidays.) They add depth behind Smith and Ahmad Brooks, who received his first Pro Bowl nod.

In the secondary, Tarell Brown's three-week rib injury late in the year opened the door for breakout corner Tramaine Brock. Brown is now the outside nickelback, ahead of pickups Perrish Cox and Eric Wright, rotation players who've fared well in spot duty. It's a sturdier cornerbacking group than the one that stumbled last February.

On the occasions when these personnel fluctuations have left the lineup lacking the talent to play straight "two-man" every down, Fangio implemented more zone concepts and even some blitzes. He maintained modest schematic diversity down the stretch, which paid off a few times. (Best example: The Cover 0 blitz call that resulted in NaVorro Bowman's game-sealing pick-six against the Falcons in Week 16.) While increased variety in personnel and scheme is important, it's ancillary to the defense's three core strengths:

1. A four-man rush that creates pressure with speed and power, often through a variety of twists and stunts. (Think Justin Smith crashing into two blockers while Aldon Smith loops behind him.)

2. The NFL's best inside-linebacking tandem in Bowman and Patrick Willis. Both can stone lead blockers and hunt down ballcarriers in traffic or in space; between them they can cover tight ends (Willis) and backs (Bowman) man-to-man.

3. A hard-hitting safety tandem in rookie Eric Reid and veteran Donte Whitner. Both missiles provide big-play potential. With more pieces to use on defense and more creative schemes, this team is stronger than it was a year ago when it came within one play of a title.


Sunday 1:05 p.m. EST


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