The conventional wisdom on roster building would suggest that the Washington Redskins, with quarterback Robert Griffin III tormenting NFL defensive coordinators, should beef up their offensive line and bolster their receiving corps. By improving the cast on Griffin’s side of the ball, the old-school football thinking goes, the Redskins would invest fully in what appears to be their fastest path back to prominence.

Thing is, Griffin’s first seven games have shown he can lead Washington’s offense to the end zone consistently while working with just about anyone. But he can’t stop the Redskins’ secondary from undercutting his winning efforts.

When the New York Giants’ Eli Manning and Victor Cruz teamed on a game-winning, 77-yard touchdown pass in the final two minutes last week, they not only ruined the Redskins’ impressive comeback bid. That punch-to-the-stomach play exposed the team’s biggest weakness in the most demoralizing way yet.

Washington’s secondary hasn’t simply been bad. The group has been bottom-of-the-NFL awful.

Opponents have averaged a league-high 328.4 yards passing. Of the seven quarterbacks who have faced the Redskins, none has passed for fewer than 299 yards. And when it comes to stopping big plays, the Redskins are at their worst. They’ve given up seven pass plays of 40 yards or more, tied for most in the league.

There’s no mystery about the root of the problem: lack of talent. The Redskins’ top cornerbacks, Josh Wilson and DeAngelo Hall, simply aren’t very good. Wilson and Hall are fast. You have to be just to get inside an NFL locker room. They’re hard-working professionals.

“They’ve done everything I’ve asked them to do,” defensive coordinator Jim Haslett said.

Still, they don’t get good enough results. And neither more coaching nor a scheme change will help them improve, two longtime NFC defensive coaches told me recently.

Possessing speed, practicing hard and focusing during film study isn’t enough for corners to succeed in today’s pass-first NFL. The game’s rules — defensive backs are prohibited from mauling receivers all over the field as they did in bygone eras — are designed to aid offenses.

The best cornerbacks are gifted all-around athletes who possess innate coverage instincts — they’re just usually in the right place at the right time — as well as smarts. Injured New York Jets all-pro Darrelle Revis is widely considered the best in his business. Revis sticks to receivers almost as well as Redskins Hall of Famer Darrell Green once did. Granted, most cornerbacks aren’t in Revis’s class. But there are better cornerbacks out there, either in the draft or free agency, than those at the top of the Redskins’ depth chart.

Much better safeties, too. Starting safeties Madieu Williams and Reed Doughty were expected to be backups. But Brandon Meriweather hasn’t played all season after suffering a knee injury, and Tanard Jackson is serving a suspension of at least a year for violating the NFL’s substance abuse policy. Jackson had missed a combined 20 games the previous two seasons because of drug use, so the Redskins shouldn’t have counted on him. In his ninth season, Williams has too often been several steps too slow while failing to provide cornerbacks with coverage help (he sure didn’t have Wilson’s back on the bomb to Cruz).

The Redskins were relying on Oshiomogho Atogwe and LaRon Landry to form a standout safety tandem. But Atogwe was released after being a free agent bust last season and the Redskins finally gave up on the oft-injured Landry, though he has played well for the Jets. Needing to start over at safety, the team scrapped the idea of adding substantial help in the secondary and cut short its free agent shopping early last offseason after losing $36 million in cap space over two years when the NFL objected to how the Redskins structured players’ contracts in the past.

In fairness to the Redskins’ defensive backs, perhaps the group would be a little better with a stronger pass rush. At least quarterbacks wouldn’t have time to make a sandwich before deciding which defensive back to torch.

Outside linebacker Brian Orakpo, who led Washington in sacks the past three seasons, and starting defensive lineman Adam Carriker, whose 5.5 sacks last season were a career high, suffered season-ending injuries in a Week 2 loss to the St. Louis Rams. The Redskins have just 13 sacks, tied for 18th in the league. Regardless of the Redskins’ lack of a big-time rush, “we should still be making plays we’re not making,” Hall said. “There’s no sympathy campaign. We asked for these jobs.”

The Redskins will have $18 million less to spend on players this offseason. And they’re still in debt to St. Louis for the right to draft Griffin: Washington owes the Rams its first-round pick for the next two seasons.

With fewer resources than most teams to bring in impact players, Washington could be tempted to get the most punch for its limited dollars on offense. Griffin already gives them a great starting point.

Surely, there would be nothing wrong with the Redskins putting better blockers in front of Griffin. And speedy, sure-handed wideouts can be a quarterback’s best friends. But before all those Griffin-produced points help put the Redskins back on top, their secondary has to stop giving up so many.

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