Oh, for the days of the "Williams Wall."

Vikings fans fondly remember Pat and Kevin Williams clogging up the middle for the Vikings in the late 2000s, as Minnesota finished first in the NFL in run defense in 2006, 2007 and 2009.

Times have changed, and the NFL is a passing league more than ever. But run defense still matters, and for the Vikings that wall has been replaced by a red carpet ushering opposing running backs well past the line of scrimmage.

On the season, the Vikings are allowing 4.8 yards per carry — tied for worst in the league with the Steelers. How do we explain such a deficiency? Andrew Krammer and I tried on Tuesday's Daily Delivery podcast, but there isn't one simple answer.

The shortcut answer against the 49ers, one of the NFL's top rushing teams that bowled over the Vikings for more than 200 yards on the ground in defeating Minnesota 34-26 on Sunday, is key absences. The Vikings were missing all four starters on the defensive line, including run stuffers Michael Pierce and Dalvin Tomlinson. It was a perfect storm.

But the problems run deeper than that. Tomlinson had started all 10 previous games, but he has yet to register a tackle for a loss this season. For the year, the Vikings have 35 tackles for a loss — a far cry from the pace set by some of Mike Zimmer's best defenses. In 2017 they had 77 TFLs, and in 2015 they had 76.

And even before the 49ers game, the Vikings ranked last in the league in an ESPN metric called "run stop win rate," checking in at 26%.

Those best Zimmer defenses had corners who supported the run well and were strong tacklers (and Antoine Winfield was one of the best run-stopping corners I've ever seen when the Vikings were leading the league in run defense).

In 2017, for instance, Pro Football Focus had Trae Waynes, Xavier Rhodes and Mackensie Alexander all in the top 26 in run stopping among corners. This year, Cameron Dantzler and Patrick Peterson have good grades but Alexander and Bashaud Breeland have fared poorly against the run. Breeland is dead last in tackling among 109 corners who have played at least 250 snaps.

Next up for the Vikings? The winless Lions. But Detroit averages 4.6 yards per carry, tied for fifth-best in the league, and ran for 108 yards in almost upsetting the Vikings earlier this year.