Buffalo Bills running back Fred Jackson opens the second half against the Redskins with a 43-yard run. (Toni L. Sandys/WASHINGTON POST)

When the Washington Redskins emerged from the locker room at halftime of last Sunday’s game against Buffalo, they knew two things: The Bills would get the ball to start the third quarter, and the defense needed — badly needed — a stop. On the first play from scrimmage, Buffalo handed the ball to tailback Fred Jackson, who ran over the left side, a simple, predictable play for a team trying to preserve a 13-0 lead.

Linebacker Keyaron Fox — a veteran reserve playing in place of Rocky McIntosh, who had suffered a slight ankle injury — tried to make a read. But Fox had spent much of the week practicing at the other inside linebacker’s spot because captain London Fletcher had been dealing with a hamstring issue. Fox didn’t get to the proper gap, Jackson scampered through the hole, and the Bills’ game-sealing drive began with an emphatic 43-yard run.

“Takes one guy,” Coach Mike Shanahan said. “One missed tackle.”

That one play provided a good chunk of Jackson’s 120-yard day, overshadowed what defensive coordinator Jim Haslett felt was a good performance by his unit, and left Redskins fans wondering: “Why can’t this team stop the run?”

“It’s one of those deals where if [Fox] probably would’ve gotten a couple of reps there,” Haslett said, “they wouldn’t have gotten that run on us.”

In the NFL, there is an explanation for everything — why one play worked or didn’t, who was responsible for what, how an opposing player found himself running unhindered. And while the Redskins’ offense has been forced to do its share of explaining in the days after the 23-0 debacle against Buffalo, the defense has been left to articulate why, in three consecutive losses, it has allowed an average of 156.5 yards a game on the ground — all with the specter of the run-first San Francisco 49ers (6-1) arriving this Sunday.

“You got to take it as a whole,” Haslett said. “You’d like to do better. We’d like to do better in all phases. The long run was disappointing last week to me. It’s been a series of things. The week before was all the different stuff [run by Carolina]. The week before that, we missed too many tackles” against Philadelphia.

The result: The Redskins are allowing 120.4 yards per game on the ground, an average that ranks 21st in the NFL (and is slightly better than the 127.6 yards per game Washington allowed a year ago, which was 26th). The downward trend — allowing 192, 175 and 138 yards in losses to Philadelphia, Carolina and Buffalo, respectively — has come against top-10 offenses. But the challenge changes little this week: Veteran Frank Gore is the NFL’s fifth-leading rusher. Over the past five weeks of the season, no team has averaged more than San Francisco’s 188.5 yards rushing per game.

“It’s about all of us being accountable and doing our jobs,” defensive end Stephen Bowen said. “We were able to do it. Everybody seemed to do it at the beginning of the year. We just got to get back to doing it.”

In a 2-0 start, the Redskins allowed the New York Giants — long known as a powerful running team — and the Arizona Cardinals just 84 yards rushing per game. The defense, which ranked next-to-last in yards allowed a year ago, appeared on the rise. After an Oct. 2 victory at St. Louis — where marquee back Steven Jackson and the Rams were limited to 45 yards on the ground — Shanahan called the defense’s performance the best since he arrived in Washington.

But Fred Jackson’s run to open the second half for Buffalo is instructive. Of the Bills’ 32 rushing attempts (excluding a game-ending kneel-down), 21 went for four or fewer yards. Only four gained more than six yards — one for seven, one for nine, one for 13. But there remains the knee-buckling 43-yarder, an error that was amplified because of the offense’s woeful state.

“We’ve got an obligation to this football team to shut people out, or keep people off the scoreboard,” Haslett said. “. . . If you’re struggling on one side [of the ball], and you give up a 43-yard run, it’s like the world’s coming to an end. . . . Whereas another team, if you’re scoring a bunch of points, you give up a long run, it’s no big deal, because it kind of gets washed under the rug.

“Our guys know that. But our guys understand that we’re in this together, and we got to do whatever we can to get turnovers, to try to help our offense score points until they get back on track.”

There also have been situational failures. On Oct. 15 against the Eagles, the Redskins were down by a touchdown with 2 minutes 44 seconds remaining. A defensive stop, and Washington had a chance. But on first down, LeSean McCoy gained 11 yards. On the next play, he gained seven. Two plays later, on third and one, quarterback Michael Vick sneaked for a first down. The Redskins knew the Eagles would run, and they couldn’t stop it. Ballgame.

“For the most part, it’s all about ‘want-to,’ I feel like,” rookie outside linebacker Ryan Kerrigan said. “You’ve got to be able to come up with a stop in situations like that.”

The Redskins’ next opportunity comes against the 49ers, who allow all of 73.4 yards per game on the ground, best in the league. They are consistently productive with very few breakdowns. That’s not Washington’s story.

“We’ve played well at times, and we’ve been inconsistent at times,” Shanahan said. “What you want to do is you want to keep up that consistency. When you’re going against a great back, you have to be on point the whole game, because if one guy misses a responsibility, then all of a sudden there’s 50, 60 yards.”

And all of a sudden, a problem that another team might be able to overcome becomes one that has contributed mightily to the Redskins’ current skid.