Rookie defensive backs Bacarri Rambo, David Amerson and Phillip Thomas are inspiring hope for the Washington Redskins. That’s a lot more than Washington’s safeties and cornerbacks provided last season.

From the opening practice here, Rambo and Amerson have been training camp standouts. Thomas has taken steps forward recently. They’re the biggest reasons why the Redskins, who gave up the NFL’s third-most passing yards in 2012, expect to show major improvement in pass defense. We’re about to find out whether a turnaround will happen quickly.

The rookies are scheduled to start Thursday night in Washington’s preseason opener at Tennessee. Coach Mike Shanahan and defensive coordinator Jim Haslett are eager to evaluate them in a game. But based on the early returns, Rambo, Amerson and Thomas appear to be a winning combination for the Redskins, who needed a new look in their secondary.

Last season, the Redskins had arguably the worst collection of safeties in the league. Cornerback was a problem as well. Although the group showed improvement during Washington’s seven-game, season-closing run to the NFC East title, the bad show was appropriately canceled.

Shanahan turned to the draft for help, selecting Amerson in the second round, Thomas in the fourth and Rambo in the sixth. It seems the Redskins chose well.

The Post Sports Live crew previews the Redskins first preseason game against the Tennessee Titans. (The Washington Post)

Rambo has claimed the starting safety spot, impressing coaches with his speed and playmaking ability. When the ball is in the air, Rambo acts as if it belongs to him. The person he replaced, Madieu Williams, was 31 years old last season but played as if he were 60.

Often on deep balls, Williams was several steps behind receivers. Rambo enjoys being challenged because “it gives you a chance to make plays. They didn’t bring me here to watch.” Rambo covers so much ground so fast, Redskins quarterbacks have been cautious about throwing downfield.

They also think twice about throwing in Amerson’s direction. Redskins people will tell you he is the most talented cornerback on the team. I know some of you are probably thinking, “That’s not saying much.”

Without a doubt, veterans DeAngelo Hall and Josh Wilson were better at complaining to officials than covering receivers. For the past few seasons, Hall has been a target of frustrated fans because of his unfortunate knack for giving up big plays.

But Amerson’s skills are worth noting on their own merit. He jams receivers effectively, displays impressive technique in covering every route and possesses the speed to recover if he gets beat. Currently the third cornerback, Amerson is essentially a starter because NFL teams use many multiple-receiver sets.

Throughout camp, Amerson has fit in well with the first-team defense. If he has felt overmatched, it hasn’t showed.

“None of us came here to sit on the bench,” Amerson said. “You come in trying to make plays and showing you’re ready for anything. . . . We’re all thinking the same way: We want to start.”

Thomas appears to have a tougher path to the first string. With two rookie defensive backs already in prominent roles, Redskins coaches would prefer to have experienced safety Brandon Meriweather, limited to one game last season because of injuries, on the field as much as possible. However, Meriweather’s surgically repaired right knee hasn’t cooperated in camp and he has missed a lot of practice time.

Thomas is a big hitter who has become increasingly assertive. If he continues to make a move during the preseason, Shanahan and Haslett might consider starting three rookies in the secondary, which hasn’t happened often. Hall of Famer Ronnie Lott was part of a group that had the experience.

Arguably the greatest defensive back in NFL history, Lott joined fellow rookies Eric Wright and Carlton Williamson as starters in San Francisco’s secondary in 1981. San Francisco had the NFL’s second-ranked defense and the team won the Super Bowl.

There’s so much rookie defensive backs must learn to succeed in the NFL, “and the toughest thing is just not making mistakes,” Lott said in a phone interview this week. “Knowing Mike [Shanahan] as well as I do, I know the young guys have ability or Mike wouldn’t have drafted them. But can they take everything they’ve been learning and line up and play good football for 60 plays right away? That’s the main question.”

Offensive coordinators won’t cut them any breaks.

Beginning with Thursday’s exhibition game against the Titans, Rambo, Amerson and Thomas should expect play-callers to “go right after them to see what they’ve got,” Lott said. “Every team is going to come up with a way to try to create a mismatch to try to confuse them.

“If they can figure out what they need to know and then just go play without [overthinking], the Redskins will have a great situation where they could grow with these young guys. That’s really what you want.”

For some time, the Redskins have struggled to assemble an effective secondary. Finally, they just may have the right parts.

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