Alex Smith's season-ending injury has placed pressure on the Redskins defense, as their offense has not been able to sustain long drives the way it did before. (Photo by John McDonnell/The Washington Post)

When Alex Smith was rolled off FedEx Field on a cart that Sunday four weeks ago, who knew how much his broken leg would decimate not just the Redskins' offense but their defense too? Their coach, Jay Gruden, is sure of this. So are some of their offensive players.

Washington had followed a safe and predictable formula to first place in the NFC East this fall. As the Redskins quarterback, Smith slowly maneuvered the downfield by mixing quick throws with handoffs to Adrian Peterson. Time drained from the clock. Some of his drives lasted for more than five minutes. And even if Washington didn’t score, its offense had rested the defensive players.

In the games after Smith left, the Redskins have lost that gentle balance of a methodical, controlling offense that built early leads, churned through time and let a fresh, physical defense hold on to win. The defense looks weary, missing tackles, giving up more and more big plays.

“We’ve been playing behind a lot, which wears down a defense — they’ve been on the field a lot — and the entire playbook has been open for the other team,” Gruden said. “When we were ahead, when we were successful, teams were throwing more and we had our pass rush going and had a lot of sacks and pressure, a lot more success. [Now] teams are lining up in two tight ends, three tight ends and then spreading you out.”

Because those other teams no longer have to throw to get back in games, they can mix runs with short passes the way Smith did in September and October, forcing Washington’s defense to stay on the field longer.

“As an offense we have been having them on the field a whole lot, and any defense — I don’t care who you are, if it’s Chicago’s defense out on the field for 40 minutes they’re going to get tired,” running back Chris Thompson said.

In the weeks since Smith’s injury, the Redskins defense has collapsed, allowing an average of 414 yards over the past three games as opposed to 337 yards before he broke his leg. The team’s average time of possession was 30 minutes per game the first 10 games; it’s now down to 28.

While the loss of Smith on Nov. 18 was a devastating blow for Washington’s defense, the unit had been faltering even before his injury. The previous week, Washington allowed 501 yards at Tampa Bay, only winning the game by virtue of four Buccaneer turnovers and two missed field goals. But the big defensive plays have not come in the past few weeks. The defense forced 21 turnovers in the team’s first 10 games, yet only two in the past three.

Still, bigger questions than the offense’s struggles are to blame for the defense’s collapse. Around the Redskins, no one seems to have a clear answer as to why a defense that carried the team through the season’s early weeks has turned into a weakness.

Throughout the fall, both players and coaches have talked about “communication” problems that have led to opposing receivers winding up wide open. A few players have hinted at midseason changes in the way the defense is structured, something linebacker Zach Brown grumbled about when he said earlier this week: “They changed stuff. If it’s not broke don’t fix it.” Safety Ha Ha Clinton Dix, who was supposed to combine with D.J. Swearinger to give the Redskins one of the NFL’s best safety tandems when he was picked up in a late-October trade, has not played well. The start of the defense’s decline almost matches his arrival.

Mostly when asked, though, Washington’s players and coaches offer some version of the phrase “can’t put a finger on it” and promise that the mistakes will have to be “cleaned up.”

Privately, coaches say they have not lost faith in the defense. They still believe in the defensive front of Jonathan Allen, Daron Payne and Matt Ioannidis. They continue to rave about outside linebackers Ryan Kerrigan and Preston Smith and say that cornerback Josh Norman has played well after struggling early in the year.

Still the Redskins have not tackled well in recent weeks.

“It seems like a team that’s just rolling over and not paying attention to detail and not necessarily not giving 100 percent effort,” said Louis Riddick, an ESPN analyst and former Redskins executive, when asked about the team’s defense. “But the intensity and the consistency with which they play the game with the ferocity that they need, and that they were doing early in the season, that’s just not there.”

“I will say that the game, obviously, a big part of it is mental, a big part of it is staying motivated and having hope and remaining dialed in,” Riddick said, “and once you see your franchise quarterback go down like that like Alex did, it can be just deflating for your frame of mind and your attention to detail.”

An offseason emphasis on toughness and stopping the run carried into the season’s first two months. Through the first seven games, Washington only gave up 100 yards rushing twice (104 to Indianapolis and 100 to Green Bay). The Redskins have allowed at least that amount every week since. After holding Giants rookie running back Saquon Barkley and Dallas running back Ezekiel Elliott to under 40 yards each the first time the Redskins played them, both had big games the second time they faced Washington. Barkley had 170 yards in last Sunday’s 40-16 Giants rout of the Redskins.

“We got to keep on practicing, keep on preaching the tackling, the angles to the ball and swarming to the ball,” defensive coordinator Greg Manusky said. “I think those things eventually we have to make sure we take hold of in these next three weeks that we’re playing because you never know what’s going to happen.”

Swarming to the ball was a big part of Manusky’s defensive plan earlier in the year. It was something the Redskins did well in the season’s first week, surrounding running backs with two and three tacklers. It’s hard to know exactly why that isn’t happening now. Supporting Gruden’s theory that the offense’s limitations have forced the defense to stay on the field longer, the first 100-yard game by an opposing back against the Redskins (Elliott’s 121 on Thanksgiving) came the first game after Smith’s injury.

The coaches also have put more pressure on Allen and Payne by relying on their youth and strength, playing them on almost every down. Both have taken more than 75 percent of the defense’s snaps. And while Stacy McGee has come off injured reserve to offer a bit of a break, veteran Ziggy Hood — praised for his quiet leadership — rarely played and was let go when McGee was activated.

Both Gruden and Manusky say Allen and Payne (and even Ioannidis, who has battled injuries in recent weeks) have not worn down. And yet the defense seems strained by a lack of depth, especially in the secondary where the Redskins seem to miss cornerback Kendall Fuller, who was sent to Kansas City in the offseason trade for Smith. His replacement, Quinton Dunber, played well until a nerve injury forced him to miss several games before Dunbar was finally placed on injured reserve.

The loss of Fuller, combined with a failed free agent signing of Orlando Scandrick, cut in training camp, left the Redskins without enough experienced cornerbacks to cover for Dunbar. Fabian Moreau had to move from nickel to the outside forcing promising rookies like Greg Stroman to play nickel, a position that has a variety of coverage challenges.

"It’s a lot to ask a guy who just got here,” Gruden said.

Not long after Smith was injured, when the Redskins still had hope that quarterback Colt McCoy could continue to manage a clock-eating offense, Gruden said the final playoff spots would be decided by which teams could run the ball well on offense and stop the run on defense. In the Redskins last three games, opponents have 503 yards rushing to their 268. After realizing that a patched-together offensive line was not going to properly protect Mark Sanchez, who started last Sunday in place of the injured McCoy, Gruden’s best hope is that newly-signed quarterback Josh Johnson can scramble enough to sustain drives and give the defense time to rest.

A lot depends on the success of this, including, perhaps, the jobs of Gruden, Manusky and the other coaches. But the future of the Redskins defense may rest in what happens these last three games. Coaches continue to believe in the foundation built by Kerrigan, Allen, Payne, Ioannidis and Dunbar. Smith is a free agent who could easily get more than $10 million a year as a pass rusher in his mid-20s.

Given recent comments from inside linebackers Brown and those attributed to Mason Foster on a social media account of his, neither Brown nor Foster expect to be back next season despite having years remaining on their contracts. Norman, who will take up nine percent of the team’s salary cap next year, could be released or traded if the team decides to go younger.

The reason the Redskins claimed middle linebacker Reuben Foster off waivers, despite the player’s arrest on domestic violence charges, is they imagine they will get a young, fast linebacker at a bargain price should the NFL clear him to play next season. But even if the team does keep Reuben Foster, the last weeks have shown that it needs more depth, especially in the secondary.

Those decisions are for the winter, however. The biggest issue for the Redskins current defense is the remaining three games and the playoffs that remain possible despite an all-out collapse.

“Iust overall consistency of swarming to the ball is major in the National Football League,” Manusky said. “We didn’t get it done [last Sunday]. We have to do a better job in a coaching aspect and same thing on the field. We have to make sure we get those big [running backs] down.”

They have three weeks left to see if they can.

Kareem Copeland contributed to this report.

Read more on the Redskins:

Svrluga: Stadium decision is the one Redskins can’t afford to get wrong

Ranking the Redskins' five worst losses of the Jay Gruden era

Boswell: So you want to give Daniel Snyder land for a new stadium? Don’t say you weren’t warned.

Clinton Portis says loss to Giants shows some Redskins are ‘already on vacation’

Redskins QB Josh Johnson: ‘A week ago I was home in the hood, chilling with the kids.'