For Del Rio, the first thing that jumped out to him was that the unit was comprised of "proud men" who had a disappointing run during the 2019 season. Most, if not all of the players on that side of the ball would probably agree. A defense that had such promise and hype in early September finished the season giving up 27 points and 385 yards per game.
Those numbers ranked near the bottom of the respective categories in 2019 and aren't enough to win games, but both Rivera and Del Rio saw an opportunity to turn the unit around under their leadership. The talent, youth and toughness are there to make the Redskins a dominant defensive force once again.
There would still need to be some changes, though -- the biggest of which would be switching to a 4-3 defensive front.
"I've spoken to a couple of players thus far, and their mentality is that they want to make it right," Del Rio told Voice of the Redskins Larry Michael on "Redskins Nation." "They want to work, and that's what it's going to take."
It has been a while since the Redskins have had a 4-3 defense; you have to go all the way back to the 2009 season when Jim Zorn was the head coach and Greg Blanche was the defensive coordinator. That front featured defensive tackle Albert Haynesworth, inside linebacker London Fletcher and outside linebacker Brian Orakpo.
That group also finished the year near the bottom of the league in terms of total defense. It gave up an average of 320 yards per game and had 336 points scored against them on its way to a 4-12 finish. Mike Shanahan became the head coach the following season, and one of the first moves he made was hiring Jim Haslett as his defensive coordinator to switch the Redskins to a 3-4.
The team ran a 3-4 defense for the next 10 seasons until Rivera took over with Del Rio as his defensive coordinator. Once they looked at the roster, Del Rio said the need to switch to a 4-3 was "obvious."
"I think it fits very well, I really do," Rivera said during his introductory press conference. "You look at the defensive line, and we've got guys that we believe can play the one-and-three techniques already."
One of the main differences between a 4-3 and a 3-4 front is that the defense has more defensive linemen and fewer linebackers on the field. A 3-4 front is often viewed as being the more flexible of the two partly because outside linebackers can either drop back in pass coverage or play on the line of scrimmage and blitz the quarterback.
Its flexibility, combined with NFL offenses becoming more pass-heavy, has forced more defensive coordinators to utilize the 3-4 front to better defend against the pass. That might mean a less consistent pass rush, but teams are willing to sacrifice that in order to give up fewer yards through the air.
But the pass rush is exactly why Rivera and Del Rio believe a 4-3 front is a good fit for the Redskins. The team has players like first-round pick Montez Sweat and veteran Ryan Kerrigan -- players Del Rio called "premier edge rushers" -- already in place to put pressure on opposing quarterbacks.
Sweat was second on the team with seven sacks in his rookie season, while Kerrigan is 1.5 sacks away from breaking the franchise's career sack record. Rivera and Del Rio want to see more of that.
"It just makes sense to have them covering less and rushing more," Del Rio said. "I'll keep it as simple as that."
Having a strong pass rush is only part of the equation for making a solid 4-3 front, though. While every defensive player should be dedicated to stopping the run, a 4-3 defense puts the majority of that responsibility on the linebackers.
In an ideal scenario, defensive linemen like Matt Ioannidis and Daron Payne would hold up offensive linemen from working their way up to the second level and clog up running lanes. That would allow players like Cole Holcomb and Jon Bostic -- tied for second on the team with 105 tackles last season -- to meet running backs at the line of scrimmage for either no gain or a loss.
According to Rivera, the Redskins are covered in that area as well.
"I think [the linebacking core] is a pretty solid group of guys," he said. "They run well, they play physical. What we want to do is play downhill through their gap. We're going to play the run on our way to the quarterback."
That's a good goal to have, especially when considering that Del Rio describes himself as a coach who likes a "penetrating front" that is disruptive. Teams aren't going to run the ball against the Redskins, Del Rio said, and that's the expectation, based on his history. Del Rio has finished with a top 10 rushing defense in each of his four seasons as a defensive coordinator.
"I like the linebackers to make the right fits and understand the fits," Del Rio said. "We'll understand how to fit it up. We want to make them go the hard way. It all starts up front, and we have a good foundation to get started there."
The Redskins will use a 4-3 front as their "base" set, but don't expect them to stick completely to it for the entire 2020 season. No one except Rivera and Del Rio know for sure what the Redskins will look like on defense. This is the first time they have worked together, so it's hard to say what kind of results combining their defensive minds will yield.
The best way to get a sample of defenses under Rivera's tenure is by looking back at what he did when he was the head coach of the Carolina Panthers. Panthers senior digital content writer Max Henson covered Rivera for almost the entirety of his nine years in Charlotte, North Carolina, and had several conversations with him about his defensive style and philosophy.
The biggest takeaway from those talks was that in order to be successful in the NFL, defenses have to be multiple.
"While the Panthers were a 4-3 throughout most of his time, they incorporated some odd-front looks," Henson said. "They weren't afraid to have some guys stand up and move around and try to create some confusion.
"You don't want to put yourself in a box by any means, and especially how offenses already have the upper hand, you want to have as many things in your back pocket as you can throw at them."
Henson added that Rivera is going to do whatever necessary to put his players in the best position to succeed. In the four weeks between Redskins owner Dan Snyder approaching Rivera about the position and him eventually accepting, Rivera studied six of the Redskins' games, including the last four, and made write-ups on every player.
Those write-ups showed that this defense would be best suited to work out of a 4-3 front. It has the players and tools to make the transition work, and Rivera and Del Rio are excited for the talent they have at their disposal.
But the amount of talent isn't what makes this team special, Del Rio said; the results will not be determined by potential. That will come from learning to work together and maximizing their abilities.
He's ready to get that process started.
"We've got a lot of work to do," Del Rio said. "It's going to be a process to get this team where it belongs. We're just going to roll up our sleeves and get to work."