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On paper, the Patriots defense looks like it belongs to a team that finished 6-10 in the AFC South.

There are Jabaal Sheard and Barkevious Mingo, a pair of guys the Browns had no use for. There are unwanted castoffs from the Bears (Shea McClellin), Eagles (Eric Rowe), Lions (Kyle Van Noy) and Bills (Alan Branch). Veteran pass-rushers Rob Ninkovich and Chris Long terrorized quarterbacks around the league…back in 2012.

Only three first-round picks are starting for the Patriots defense: safety Devin McCourty, linebacker Dont'a Hightower and tackle Malcom Brown. The team jettisoned two first-round defenders (Chandler Jones and Dominique Easley) in the offseason and a second-rounder (Jamie Collins) in the middle of the year. In their places are unheralded youngsters Trey Flowers (23), Vincent Valentine (22) and Elandon Roberts (22).

The Patriots made the types of personnel moves on defense that get general managers mocked on talk radio. But instead of turning into the Colts or Saints, they built their collection of youngsters and budget free agents into a playoff-caliber defense.

Eventually, anyway.

The Patriots defense was shaky in the first nine games of the season. The Dolphins rang up 457 yards against them in Week 2. The Seahawks, not known for winning with their offense this year, put up 420 yards on them. Even Steelers backup quarterback Landry Jones threw for 281 yards and a touchdown, preventing the Patriots from pulling away until late in the game.

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The Patriots defense stiffened up in the second half of the season, with no opponent gaining more than 350 yards against them since the Seahawks in Week 10.

A skeptic would point out the newfound success came against Jared Goff, Bryce Petty, Matt Moore, Trevor Siemian and others. Yes, a slate of bad quarterbacks helped. But the Patriots weren't exactly facing Aaron Rodgers and Matt Ryan early in the season, either.

Bill Belichick and his staff made some second-half adjustments to get the most out of the Patriots' no-name defense. They also stuck to some things that have worked for Belichick for more than a decade.

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Here's a look at what the Patriots are doing now, why it's working, and what the team might do Sunday to stop Ben Roethlisberger, Antonio Brown and the toughest offense they have faced all year.

A shuffle in the secondary

The biggest change the Patriots made late in the season was moving cornerback Logan Ryan to the slot, with early-season acquisition Rowe taking over as the starting right cornerback.

At 6'1", Rowe has matched up well against bigger receivers. Ryan, who struggled this season as an outside cornerback, has thrived in the slot. Ryan started on the outside for the first time in weeks against the Texans in the divisional round, but he frequently slid inside and recorded both a sack and an interception.

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"I think Logan's really helped us playing in there," Belichick told reporters after making the switch in December. "We were kind of in a little bit of a transition earlier in the year with the secondary, and Logan in particular, outside, inside. I think the last couple of weeks he's really given us a good level of communication, of run force."

The new Ryan-Rowe combination multiplies Belichick's coverage options. When the Patriots faced the Steelers during the regular season, Malcolm Butler shadowed Antonio Brown for most of the game and had decent results: Brown caught seven passes for 106 yards, but Butler intercepted a pass to Brown in the end zone.

Butler will probably cover Brown frequently Sunday. But if Brown hides in the slot, Belichick can trust Ryan more than the slot defenders (Patrick Chung and Justin Coleman) he used in the last meeting. If Belichick commits double coverage to Brown, he can count on the rest of his secondary locking down their assignments.

It helps that Belichick can often spare an extra body in coverage.

Three up front, eight in the back

The line is the strength of the Patriots defense. While officially listed as a 4-3 defense, the Patriots use a vast array of fronts, usually anchored by gap-pluggers Brown and Branch flanked by waves of situational edge players like Long, Flowers, Sheard and Ninkovich.

The Patriots also rush just three defenders on passing plays more than any other team—an NFL-leading 25 percent of the time in the regular season, according to Sports Info Solutions. It's not unusual to see the Patriots crowd the line of scrimmage with six or seven possible pass-rushers, only for all but three of them to drop into coverage.

This video clip shows Broncos quarterback Trevor Siemian getting sacked by Flowers, despite the fact that the Patriots rushed just three defenders.

Coverage is the key to the Siemian sack. Eight Patriots drop into coverage. Sheard and McClellin sit in flat zones. Van Noy occupies the middle of the field. Ryan and Butler lock down receivers in man coverage, and Rowe and Chung double-cover Demaryius Thomas with a safety deep. It almost looks like they have 13 men on the field.

The Patriots rushed three defenders against the Steelers frequently in Week 7. It's an appealing option when trying to neutralize Brown, but there is a big difference between confusing Landry Jones or Siemian with eight coverage defenders and giving Roethlisberger extra time to improvise against a three-man rush.

Even if they rush four or more defenders, the Patriots can use diverse personnel packages to disguise their intentions and deploy their speedy newcomers in places where the opponent least expects them.

Tampa 2 seasoning

One other major midseason change the Patriots made was trading Collins away for a future draft pick. Van Noy and Roberts, the young linebackers who took over when the Patriots traded Collins, are not polished players. But both can flat-out fly around the field. Belichick has taken advantage of their speed by using Tampa 2 concepts that allow his young linebackers to range around the middle of the field.

The Tampa 2 defense is a variation on the Cover 2, where two safeties cover deep zones while five defenders cover short zones and four rush the passer. In the Tampa 2, an inside linebacker races back into the deep middle of the field, creating more of a three-deep zone that allows the safeties to work closer to the sidelines.

Belichick's variations on the Tampa 2 look different than the one you see in Madden, as you might imagine. This diagram shows a zone-blitz variation on the Tampa 2 concept the Patriots used in the first quarter against the Broncos:

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The slot cornerback—Jonathan Jones (31) on the play shown, though it will probably be Ryan on Sunday—blitzes, while Ninkovich (50) stunts inside. Flowers (98) drops into zone coverage from the defensive line. Roberts (52) races deep, while Hightower (54) hustles over to occupy the "hook" zone that was vacated by the blitzing Jones. Siemian tries to hit a tiny window between Flowers and Butler (21) to connect with tight end Virgil Green, but the throw is rushed and off target.

Against the Steelers in Week 7, Belichick would sometimes send a linebacker (Roberts or Collins) to "buzz" underneath Brown or another receiver to take away short slant routes. That allowed Butler or Rowe to play with a deeper cushion. Mix the three-man rush with mobile linebackers, and the Patriots can theoretically triple-cover Brown (linebacker underneath, safety deep, cornerback in man coverage) and still have enough defenders left over for the rest of the Steelers receiving corps.

A defender for every role

As you have probably heard, Belichick is a football genius. That's never more evident than when he assembles a defense out of less than stellar talent.

Belichick never asks defenders to do what they cannot. He defines realistic roles for the available players. At the same time, he develops those players for their specialized roles. Players who were useless or limited elsewhere become specialists in support of the Patriots' small core of Pro Bowl-caliber starters like Butler, McCourty, Hightower and now Flowers.

McClellin flunked an extended tryout as a traditional 4-3 linebacker in Chicago. In New England, he's a useful situational coverage linebacker. Chung was a mess when the Eagles tried to make him a traditional safety. He's much better at covering tight ends and running backs in a dime package. Ninkovich, once the Patriots' top edge-rusher, now does most of his damage on stunts like the one shown in the diagram. Rowe and Sheard are much more effective in their clear-cut Patriots roles than they were in Philadelphia and Cleveland.

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Belichick has been building defenses this way for more than a decade. In fact, this defense looks a lot like the one the Patriots had when they won their first Super Bowl in 2001.

That 2001 team had a great cornerback in Ty Law and a great safety in Lawyer Milloy; think of them as Butler and McCourty. Aging pass-rusher Anthony Pleasant was the Chris Long of his day. Rowe replaces Otis "My Man" Smith as the much-maligned former Eagles cornerback suddenly playing well. McClellin is Mike Vrabel, who could never find a role among the Steelers linebackers but blossomed in New England. Future All-Pro Richard Seymour was a rookie on that team; Flowers looks like the young Seymour at times.

That analogy can be taken too far; Law is a heck of a lot better than Butler, for example. But this version of the Patriots' no-name defense has precedent. No one turns trash into treasure like Belichick. The Patriots defense may have been vulnerable early in the year, but this readjusted version of it is ready to handle its biggest challenge yet.

Mike Tanier covers the NFL for Bleacher Report. Follow him on Twitter: @MikeTanier.