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If Texas linebacker Derrick Johnson can take heart in anything, it's that scouts are always notorious misers with defensive evaluations.

They'll be the first to admit that finding a rookie who can score touchdowns is tricky, but finding one who can prevent them is brain surgery. Which is why guys like Johnson are so often nitpicked to death in the waning days before the draft. It's a brutal reality that has led to him sliding down draft boards, from a top-three selection in February to a top-seven selection in March to barely hanging on in the top 10 in April.

He's hardly the first. The same thing happened to Miami linebacker Jonathan Vilma a year ago. Despite leaving the Hurricanes as the best linebacker in college football, Vilma watched his resume get riddled with holes as the 2004 draft approached. According to one team scouting report, he lacked the "bulk" and "strength" to play middle linebacker, and he had "difficulty shedding blockers."

A few months after that report, Vilma, who was drafted by the New York Jets with the 12th overall pick, became the best rookie linebacker in the NFL and was being hailed as one of the league's brightest young stars. Even apparent brain surgeons lose sight of the big picture.

Tampa Bay Buccaneers coach Jon Gruden has taken that wide-screen view of Johnson's skills. "You watch [Texas'] defense on tape, and the guy is constantly in the picture," Gruden said. "He's always making things happen."

Ultimately, that's what the search for defensive prospects boils down to. The ones with the ability to shine, to make things happen, are the ones who churn out big rookie seasons like Vilma – not just starting, but starring.

That ability is what makes Johnson the leader of this season's batch of impact defenders. Below are the positions expected to produce at least one significant defensive rookie in the NFL this season, along with the likeliest player to shine.

LINEBACKER

  • Derrick Johnson, Texas
    Like Vilma last year, people are complaining about Johnson's ability to take on blockers. But as Gruden pointed out, Johnson has found a way to disrupt offenses for four years at a major program.

His speed and added bulk should suit him well at the outside linebacker spot in the NFL and make him an instant starter. Though he might not be able to put together a rookie season like Vilma did in 2004, he shouldn't be far off the numbers D.J. Williams put up in his debut with the Denver Broncos (114 total tackles, two sacks, one interception).

Florida's Channing Crowder is another linebacker likely to fill up the stat sheet. But I'd be wary about counting on major first-year production from guys being converted into the linebacker spot from other positions – like Maryland defensive end Shawne Merriman and Georgia safety Thomas Davis (if both do end up as linebackers in the NFL). Such transitions can be murder on a player's early production.

DEFENSIVE END

  • David Pollack, Georgia
    Along with cornerback, this is one of the defensive positions that takes some time for growth. Monster rookie seasons from a defensive end – like the ones posted by Dwight Freeney and Jevon Kearse – are few and far between. And this year's crop isn't exactly "pure," since some of the better defensive ends in this draft are looking at possibly being switched to outside linebacker in a 3-4 defense. Pollack, along with Maryland's Shawne Merriman, could be switched.

But there are also several 4-3 teams looking at Pollack as a traditional defensive end, and if he stays there, he has the best chance to have an instant impact.

Though he's not the biggest end (6-foot-2, 265 pounds) or the fastest (4.75 seconds in the 40-yard dash), he is the most hyperactive. And among linebackers and defensive linemen, he and Texas' Johnson were the only prospects to post sub four-second times in the cone drills at the combine, illustrating amazing acceleration for their size.

That should be enough to give Pollack the edge over other guys in the class, like LSU's Marcus Spears and Wisconsin's Erasmus James. But don't expect double-digit sack production from Pollack or any other end. It should be more in line with last year's numbers from Will Smith (7½ sacks) and Jared Allen (nine sacks).

DEFENSIVE TACKLE

  • Travis Johnson, Florida State
    It's the second straight lean year for defensive tackles in the first round. Not only is Johnson the only likely prospect to have an instant impact in the NFL, but he could also end up being the only one selected in the first round – depending on how much USC defensive tackle Shaun Cody's stock slips.

Often, defensive tackle is a "plug" position, where highly regarded prospects get thrown directly into the fire. Johnson might be the only rookie worthy of such a fate, although the major concern with him is that he isn't massive (6-4, 298) and he didn't have a tremendous college career. Essentially, he built his draft stock with one huge year at Florida State.

Some scouts think he's going to struggle when he's matched up with big guards, but Johnson showed in his workouts that he has all the quickness and speed to succeed. His game is similar to Tommie Harris, who was drafted 14th overall by the Chicago Bears last year. If healthy, Johnson's numbers should be comparable to Harris' rookie output (43 tackles, 3½ sacks).

CORNERBACK

  • Antrel Rolle, Miami
    The impact of cornerbacks is typically negative. Most of them – even touted "shutdown" corners like the Dallas Cowboys' Terence Newman and Denver's Champ Bailey – have to take their lumps for a year or two. But last season was somewhat of an anomaly, as cornerbacks all over the league dished out surprising first-year punishment. While none was dominant, rookies like the Carolina Panthers' Chris Gamble and the Houston Texans' Dunta Robinson (who each had six interceptions) had some fantastic stretches. Even a fourth-round pick like Indianapolis' Jason David (four interceptions) shined at certain junctures.

This year's class is remarkably fast – maybe faster than any in history – and it should enjoy the same measured success as last season's rookies.

Rolle's size, speed and ability to punish receivers give him the edge as the player most likely to make an impressive mark in his first year. But he's going to have plenty of competition as the top rookie corner from West Virginia's Adam Jones, Auburn's Carlos Rogers, Nebraska's Fabian Washington and Michigan's Marlin Jackson. Nobody in this group has shutdown billing, but they will create some havoc on turnover ratios as rookies, perhaps even rivaling the six interceptions notched by Robinson and Gamble last year.

SAFETY

  • Thomas Davis, Georgia
    There's no way anyone from this class can live up to the chaos produced by Washington Redskins rookie Sean Taylor last season. Taylor, like Dallas' Roy Williams before him, was both brilliant and punishing at times in his first season. This class doesn't quite have that kind of star power. However, if Davis stays at safety (several teams are projecting him as a linebacker), he might change that.

In fact, Davis is a lot like Williams – he'll annihilate wide receivers at times and can come up to the line of scrimmage to roam. He's not quite the total package that Taylor offers, but he could make a lot of noise (not to mention a few highlights) from the safety spot. Overall, Davis is simply a better player than Oklahoma's Brodney Pool (who's not as physical), and Michigan's Ernest Shazor (who's not as fast).