Ed Szczepanski-USA TODAY Sports

Over the past two games, the San Francisco 49ers have had four would-be defensive starters watching from the sideline or at home. Players that are no slouches either. Patrick Willis, Aldon Smith, Chris Culliver and Ian Williams were meant to be strong points at each level of the team's defensive infrastructure, but were lost for one reason or another.

That being the case, coach Jim Harbaugh has had to summon several unknowns from his bench as fill-ins for the 2013 season. Some of these unheralded players will be taking over for the rest of the season, some are demonstrating their value as backups, and others are taking the job opportunity and running with it.

Since all of these members have been in the lineup together, and San Francisco has been fielding a shell of its starting defense, the 49ers have gone 2-0. The unit is playing hard, fundamental football, with a surprising cohesiveness given the lack of playing time together. It seems that several superstars being out of the lineup has got numerous players operating at 110 percent capacity.

For those that aren't familiar with these recent NFL success stories, here is a proper introduction to the five men that have answered the call of duty.

Michael Wilhoite

Position: Inside Linebacker

Replacing: Patrick Willis

Duration: N/A

In his first career start versus the St. Louis Rams in Week 4, Wilhoite went all out and accrued a team-high seven tackles. Against Matt Schaub and the Houston Texans, he channeled his beast mode again, finishing with 13 tackles, three tackles for loss and one pass deflection, and was also in on a sack, of which he was not credited for.

Harry How/Getty Images

In his only two starts on the season, Wilhoite has led the team in tackles, even with All-Pro LB NaVorro Bowman still in the rotation. It is staggering that Patrick Willis can be out of the lineup and there is virtually no drop off in performance from the 49ers linebacking corps as a whole.

Even though not nearly as gifted, Wilhoite has been just as aggressive, attacking the run and trying to cut the play off at the head. At a big broad, 6’0”, 240 pounds, the young safety-turned-linebacker really moves well for his size and knows how to throw his weight into his target.

So far Wilhoite has not been a liability in coverage, which may partially have to do with his background as a defensive back. And surprisingly, he also has been an intuitive sideline-to-sideline tracker, using his speed, vision and proper angles to put himself in the middle of the play.

Watch the third-year linebacker sniff out and run down the electric Tavon Austin versus the Rams two weeks ago.

Regardless of his label as a wide receiver, Austin lining up in the backfield draws the eyes of Wilhoite, who is peering behind the line of scrimmage before the play is snapped. He is watching to see if Austin takes a handoff, or perhaps a short pass, like a screen or a quick hit to the flat.

It doesn't take Wilhoite long to find out. Right off the snap, quarterback Sam Bradford dumps it down to the high-profile track star from West Virginia.

Austin takes the short pass, and by design, he's got blockers in front.

However, Wilhoite plays off his own defenders and knows where Austin is going before he does, filling into the nearest cut lane and squaring up with the ball carrier. He circumvents the traffic and gets there in a hurry, without a moment of hesitation. Just then, the 5'8", 170-pounder is like a baby deer caught in the headlights, realizing he's boxed in with no room to escape.

Wilhoite doesn't back down either, or wait for Austin to make the first move and blow by him. Like the king of the jungle, the 49ers linebacker pounces and finishes the play.

Dan Skuta and Corey Lemonier

Position: Outside Linebacker

Replacing: Aldon Smith

Duration: N/A

A regular odd couple, veteran Dan Skuta has been the go-to LB in the base defense, while rookie edge rush specialist Corey Lemonier has been the primary outside backer supplementing the pass-rush productivity left by Aldon Smith.

Lemonier has largely contributed on third down and in obvious passing situations, allowing him to pin his ears back and do what he does best, which is getting down hill and blowing up the pocket. And he’s been effective at that, considering the circumstances.

But the performances by these two linebackers should not come as a total shock.

Even if you are inclined to believe that the preseason does not mean a thing, both Skuta and Lemonier flashed in those games, showing enough to send Cam Johnson and Parys Haralson out of town. In fact, Lemonier generated pressure on over 20 percent of his pass rushes in that time, via Jeff Deeney of PFF, while Skuta also had three sacks, which tied him for fifth.

They’ve picked it up in the regular season, too.

According to Pete Damilatis of Pro Football Focus, Skuta and Lemonier combined for six pressures and three runs stops versus the St. Louis Rams, functioning as key contributors in what was the team’s bounce-back win and first game in 2013 without its All-Pro rush linebacker.

On Sunday night, in their second go around, they were just as effective, as Lemonier had one quarterback hit, two hurries and earned yet another positive grade from the stat wizards at Pro Football Focus. Skuta also pitched in with three tackles of his own, including one for a loss.

Overall, Skuta has been surprisingly reliable against the run and steady in coverage—just not an ideal edge-setting linebacker. He is not very big or overly powerful. But, as a versatile type, he can slip through traffic and make a play in the backfield, which is complementary to Lemonier, who is a lot like a wrecking ball.

Coming out of the University of Auburn in 2013, the 49ers rookie was recognized for his pass-rush ability, which, to this point, has earned him more reps at OLB, via Eric Branch of the San Francisco Chronicle. He is still pining for his first career sack but that should inevitably transpire in the near future, especially given all the near misses he’s had so far.

Glenn Dorsey

Scott Rovak-USA TODAY Sports

Position: Defensive Tackle

Replacing: Ian Williams

Duration: Indefinitely

In Weeks 3-4, replacing Ian Williams, 49ers backup defensive tackle Glenn Dorsey had the second-highest grade at his position versus the run, per PFF. He also had a league-high eight run stops and two sacks in that time. In his latest game versus Houston, he had no tackles or stops, but did have two QB hurries.

Not to mention, Dorsey was also busy clogging running lanes, which allowed inside linebackers Michael Wilhoite and NaVorro Bowman to combine for 26 tackles, including three behind the line of scrimmage.

The former first-round pick (No. 5 overall in 2008), recently cast off by the Kansas City Chiefs, never evolved into the iconic lineman he was made out to be when he was first drafted, but Dorsey has proven to be a very effective starter in this league, and that truth has carried over to Vic Fangio’s 3-4 system.

Believe it or not, he currently holds the highest grade of any player on the 49ers defense, with a booming +8.1, via PFF. Bearing in mind how the site’s stat-keepers measure NFL performances, it says something about Dorsey’s proficient style of play and the consistency of which he does it.

He impacts plays, especially versus the run. It's also worth noting that Dorsey already has twice as many sacks this season as former starting nose tackle Isaac Sopoaga did in his last two years in San Francisco combined. But outside the box score, player ratings and persona, let’s take a look at what makes Dorsey so valuable.

Personnel: 11 (1 TE-3 WR-1 RB)

Formation: Singleback

Down and Distance: 1st-and-10

Quarter: 1st

Quarterback Sam Bradford is going to hand off to running back Daryl Richardson on stretch play to the strong side. In this frame, it could very well still be a play-action pass, but the 49ers are reading the blocking scheme, which is leaning forward in attack mode, rather than sitting back in a protective stance.

On the San Francisco defensive line, nose tackle Glenn Dorsey (yellow circle) is going to engage the center and crash down against the run, along with ends Justin Smith and Ray McDonald, who are holding up the edges.

Even at 6’1”, 297 pounds, Dorsey plays bigger than his size, really using his width and big, beefy limbs to his advantage. He takes up a lot of space and is generally disruptive working up and down the line of scrimmage. In this play, he is the reason for both the Rams left guard and center being on the ground.

This basically wins the play here for the Niners, as Dorsey opens the floodgates.

With Justin Smith closing in, NaVorro Bowman and Michael Wilhoite in the vicinity, and Ahmad Brooks setting the edge and ready to break off the tight end, Richardson suddenly has a wall of defenders where the run was directed.

Richardson is looking to get the most out of this one, hoping he can find a crack, but the 49ers defenders only clamp down further.

And just like that, it’s a done deal. Dorsey’s smooth glide down the line, while absorbing and effectively removing 40 percent of the St. Louis offensive line, opened this play up for the 49ers run defenders. While Dorsey has nothing to show for it in the box score, he is the one that killed this play for the Rams.

Furthermore, this particular example was chosen because it would’ve been too easy to show a run stop by Dorsey, when in fact, his impact goes way beyond that.

Tramaine Brock

Jeff Gross/Getty Images

Position: Cornerback

Replacing: Chris Culliver, Nnamdi Asomugha

Duration: Indefinitely

According to Jeff Deeney of Pro Football Focus, 49ers cornerback Tramaine Brock is allowing a QB rating of 21.6 when targeted.

On Sunday versus the Texans, he finished with a +2.8 grade in 37 snaps, via the PFF rating system, having only allowed two catches for 32 yards receiving, snatching two interceptions, one pick-six and a pass deflection (which was nullified by an offensive pass interference).

It has been a strong start for Brock, as a lot of folks are now learning his name, but he is no stranger to the 49ers. In fact, he is one of the select few lasting members from the old regime, having joined San Francisco in 2010 as an undrafted free agent. He managed to stick around and is actually a veteran, for those who didn’t know.

For the bulk of his career, Brock has been the third, fourth and even fifth cornerback on the roster, playing in 35 career games, including 32 since 2011.

Now a fourth-year pro, well versed in this defense, Brock is convincing the world that he is ready to be a top-three featured corner in this defense, which is top-five versus the pass this year. He brought stability to a unit that was still looking for answers after losing Chris Culliver (ACL) and seeing uninspired play from a half-healthy Nnamdi Asomugha.

It has been a statement by Brock these past two weeks.

Honestly, with the wide receiver woes, people were beginning to forget that the 49ers needed a solution at the third cornerback position, especially if they were serious about a Super Bowl run this year. Well, it appears as if Tramaine Brock is the solution to that problem.

And who knows? Since he is playing out a contract year and Trent Baalke will be looking to rebuild the secondary in 2014 (via Spotrac), perhaps there is a place for the homegrown corner here in the Bay Area. All the more reason for him to keep it up.

Let’s take a look at how Brock has been making waves so far.

Personnel: 20 (2 TE-3 WR-0 RB)

Formation: Shotgun, Split Backs

Down and Distance: 3rd-and-6

Quarter: 1st

Prior to the snap, two of three wide receivers, Keshawn Martin and DeAndre Hopkins, line up on the weak side of the formation. Shortly after the 49ers counter that with their nickel package—in what appears to be man-to-man coverage across the board—Schaub signals Martin in motion.

From the left slot position, Martin slides across to the formation to the Z-receiver position, outside star wideout Andre Johnson. 49ers cornerback Carlos Rogers follows, reaffirming the man-to-man coverage in Shaub’s mind.

Johnson, lined up in the slot, is going to run a stick route, with Martin to his right on the perimeter, running an out route. Thinking it is man coverage, the Texans quarterback wants to take advantage of the route by Martin, who should be clearing that side out for Johnson to make the quick catch and turn up field for yards.

Except, after the offensive formation was set and Schaub was ready to hike the ball, Carlos Rogers and Tramaine Brock subtly switched assignments. Rogers stayed in the slot, switching to Johnson, and Brock picked up Martin outside.

Again, Schaub did not think much of it, assuming it was a quick adjustment by the defense as not to get caught by the quick snap.

Off the snap, Brock and Rogers give the pair of receivers a little bump to really sell the man coverage—just enough to get Matt Schaub to cock back and commit to the outside throw to Andre Johnson.

While it looks like the corners are out of position, they are exactly where they need to be. That window of time where it looks like they’re out of place is enough to bait Schaub into the throw just as the corners are positioning themselves in zone.

Brock is designed to cover underneath (and clearly sees No. 80 coming his way), while Rogers is making sure he assists his fellow corner with any tackle that needs to be made, as well as playing whichever receiver slips behind Brock, and potentially passing him to the safety.

49ers free safety Eric Reid (purple) is also playing that overly stacked side of the field, in what looks to be Cover 2.

The ball is in the air as Schaub commits to Johnson on the stick route. This becomes an awfully risky throw because he is a hair late on the trigger, and the man coverage he thinks Brock is in, actually turns out to be zone in the flats, which is perfecting for disrupting the particular pattern Schaub is trying to hit.

Brock reads it the entire way, drives on the underneath route, competing for the ball with way more tenacity than Johnson.

By playing a fearless, intuitive brand of football, as well as understanding the team’s defensive scheme and knowing where to take chances, Brock ends up making a house call on this play.

Watch it happen:

Statistics courtesy of ESPN.com Game Center and Pro Football Reference. Special thanks to Jeff Deeney of Pro Football Focus for providing in-depth numbers. Screen grabs taken from NFL.com Game Rewind (subscription required).

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