A realization hit defensive coordinator Dean Pees when he talked to coach John Harbaugh about the Baltimore Ravens' depth chart this offseason: Only two starters remain from the defense that helped the Ravens win the Super Bowl 17 months ago.

"When you have the [roster] turnover, there’s always a little bit of time for those guys to develop," said Pees, whose only championship holdovers are Terrell Suggs and Haloti Ngata."But also, just like every team goes through it, you can’t keep the same guys forever."

Change often leads to a transition period especially when you're dealing with the loss of two future Hall of Fame players in Ray Lewis and Ed Reed. But change was a necessity for the Ravens. For the first time in 15 years, the Ravens went consecutive seasons without having a top-10 defense. The once-feared group suddenly had become average.

The Ravens needed to get younger, faster and, if their projections are correct, significantly better. A major investment in defense -- their top three draft picks in 2013 and 2014 came on that side of the ball -- has brought an infusion of talent. Four of these players (strong safety Matt Elam, nose tackle Brandon Williams, linebacker C.J. Mosley and free safety Terrence Brooks) have a shot at starting this season.

The Ravens are mixing this youth with two of the best pass-rushers in the league in Suggs and Elvis Dumervil, a couple of emerging cornerbacks in Lardarius Webb and Jimmy Smith and a "game-wrecker" in the middle with Ngata.

"The expectation for our defense is to be top-five, at the worst," Harbaugh said. "It has always been that way and always will be."

Long before the Seattle Seahawks won a Super Bowl with defense, the Ravens did so in 2000. Then, from 2003 to 2011, the Ravens boasted a top-10 defense. Dominating defenses became as synonymous with Baltimore as "The Wire."

That streak ended in 2012, when the Ravens defense finished 17th in yards allowed and 12th in points given up. The defense played a integral role in beating the San Francisco 49ers in the Super Bowl, especially that final stand in the red zone, but that doesn't erase the fact that the Ravens gave up the second-most yards in team history that year.

The lapses continued last season, when the Ravens ranked No. 12 on defense. If not for the offense finishing near the bottom of the league and Joe Flacco's career-worst 22 interceptions, there would've been more complaining about the defense allowing the most fourth-quarter points in team history.

"Last year, at times, we showed flashes of being a good defense, but then we'd have breakdowns," defensive end Chris Canty said. "We'd have mistakes, we'd have mental errors and those are the things that just can't happen if you want to be successful in the National Football League. ... We have to make sure that we're on top of our game every single play."

Some may suggest the Ravens made mistakes in this year's draft. Based on how the offense struggled last year, it was more of a priority to add a right tackle, wide receiver or running back early in the draft.

After the Ravens chose three defensive players with their first three picks, owner Steve Bisciotti reportedly turned to new offensive coordinator Gary Kubiak and said: “That’s what we do here. We ask you for your opinion, but then Ozzie [Newsome, general manager] always takes defensive players.”

Although some bemoan what the Ravens didn't address in the draft, the focus should be on the talent they brought in. They have a future Pro Bowl centerpiece in Mosley, an eventual replacement to Ngata in Timmy Jernigan and a speedy free safety in Brooks.

"A young defense is a good thing. I'm excited about it," Harbaugh said. "When we won the Super Bowl, we definitely weren't the fastest defense in the NFL but we had a lot of savvy and had guys who made plays when it counted. What we're lacking in experience, we're going to have to make up for in youthful vigor and speed."

When the Ravens won the Super Bowl, the average age of the starting defense was 29.5 years old. This year's projected starters on defense are nearly 3 years younger.

Now they have to prove they're better.

"We can be really good," Dumervil said. "I think we have the speed and athleticism. We're very versatile. We just have to continue to grind and continue to work, and the sky is the limit."