Before the 49ers visited the Saints earlier this month, they had allowed only six 100-yard rushers in their past 78 games dating to November 2009.

During that stretch, only four different running backs had topped 100 yards – Seattle’s Marshawn Lynch did so three times – and none of those backs gained more than 118 yards.

In their past three games, however, New Orleans’ Mark Ingram (120 yards) and Washington’s Alfred Morris (125) have both gouged the 49ers’ vaunted run defense. In the Niners’ 17-13 win over Washington on Sunday, Morris had the most rushing yards against the 49ers since Green Bay’s Ryan Grant had 129 on Nov. 22, 2009.

What’s going on? The obvious answer is there’s a void in the middle: The 49ers are currently without nose tackles Ian Williams and Glenn Dorsey and All-Pro inside linebackers Patrick Willis and NaVorro Bowman.

“We definitely miss those guys playing, but we definitely have the guys that can step up and fill those spots until they’re able to return,” safety Eric Reid said. “I think the plays that got us (Sunday) – it was one guy maybe not being in the right gap. Or it’s making a slight mental error and getting out-leveraged and that creates a big enough hole for the back to get through. I think it’s all things we can get corrected. You definitely miss Pat and NaVorro. But we’ll be able to do all right until they come back.”

Of course, Willis (toe), who is on injured reserve, isn’t returning until 2015. And it’s possible Bowman won’t play this season because of his surgically repaired left knee.

In addition, Williams (fibula) is on IR and his injury in the second quarter against the Saints coincided with the recent slide. At halftime, Ingram had 35 yards on nine carries (3.9 yards a carry), but he finished with 85 yards on 18 carries (4.7) in the second half and overtime.

According to Pro Football Focus’ grading system, Williams (+12.9) ranks 11th among 78 nose tackles and defensive tackles. Meanwhile, his replacement, Quinton Dial, has earned a -2.0 grade.

The 49ers could get a boost in the middle with the return of Dorsey, who has yet to fully regain his strength after undergoing surgery for a torn biceps in training camp. On Sunday, Pro Football Talk reported Dorsey remains “a few weeks away” from being able to play.

The 49ers’ run defense, which still ranks seventh in the NFL, has to be ready Thursday when Lynch, their longtime nemesis, arrives. Lynch has accounted for four of the 12 100-yard rushing performances the 49ers have allowed in their past 95 regular-season games.

Here’s a look:

11-23-14: Alfred Morris (Washington): 21 carries, 125 yards

11-9-14: Mark Ingram (Saints): 27 carries, 120 yards

9-7-14: DeMarco Murray (Cowboys): 22 carries, 118 yards

12-23-12: Marshawn Lynch (Seahawks), 26 carries, 111 yards

11-11-12: Steven Jackson (Rams), 29 carries, 101 yards

10-18-12: Marshawn Lynch (Seahawks), 19 carries, 103 yards

10-14-12: Ahmad Bradshaw (Giants), 27 carries, 116 yards

12-24-11: Marshawn Lynch (Seahawks), 21 carries, 107 yards

11-22-09: Ryan Grant (Packers), 21 carries, 129 yards

11-8-09: Chris Johnson (Titans), 25 carries, 135 yards

12-21-08: Steven Jackson (Rams), 32 carries, 108 yards

11-30-08: Marshawn Lynch (Bills), 16 carries, 134 yards

Twitter: @Eric_Branch