While Minnesota Vikings fans lament Christian Ponder's slump, fret over Percy Harvin's badly sprained ankle and rip the play calling of offensive coordinator Bill Musgrave, another significant concern exists on the other side of the ball. The Vikings run defense -- ranked among the best in the league a month ago -- has been obliterated the last four games.

The Vikings have now allowed a running back to top 100 yards and score against them in three straight games: LaRod Stephens-Howling, Doug Martin and Marshawn Lynch. Prior to this recent funk, they hadn't allowed three straight running backs to top 100 yards rushing since 2004 and hadn't allowed three straight running backs to top 100 yards rushing and score a touchdown against them since 2003. Lest we forget the week before Howling registered his first career 100-yard game against the Vikes, Redskins rookie quarterback Robert Griffin III rushed for 138 yards and two touchdowns against them. Through their first five games of the season, the Vikings allowed an impressive 3.2 yards-per-carry – the second-best rate in the NFL – and did not permit a single running back touchdown. Altogether through five games they were ceding 78.6 rushing yards per game and had allowed just one rushing touchdown (to Lions quarterback Matthew Stafford). However, over the course of the last four games the Vikings have allowed 4.6 yards per carry. Their yards-per-game allowed has more than doubled to 165.8 yards per game. And they have allowed six more rushing touchdowns. So, yeah, the Vikings' run defense is in just as much of a slump as their pass offense. If there were injuries to the front seven it might be somewhat understandable, but there aren't. Head coach Leslie Frazier kicked off his Monday press conference a few days ago with a rather scathing assessment of his team's ability to stop the run lately. "Defensively, our run defense doesn't resemble the type of run defense we're capable of playing" Frazier said. "We have to go back to the drawing board and come up with some ways to defend the run better than what we're doing. We've got to look at some things and try to get things corrected and really almost start from square one when it comes to our rush defense. It's not up to par by any means." The Vikings' run defense "drawing board" had better elicit some answers in short order – not so much for this weekend's game against the pass-happy Lions, but for what lies ahead of them after the bye week. Three of the Vikings' final six games come against offenses that can and will run the ball all day if allowed, with two games against the Bears and one against the Texans. Matt Forte and Arian Foster are poised to pick up where Martin and Lynch left off unless something is done; and if they do, the Vikings won't have a chance of winning those games. They'll likely look better stopping the run against the Lions this Sunday. Detroit ranks 22nd in rushing offense at 103.6 yards per game and in their first matchup with the Vikings this season, back in Week 4, they managed only 55 yards on 20 carries. Even so, the Vikings would be wise not to relax if they find run defense success for one week against Mikel Leshoure and the Lions. Whatever the answer, be it execution, scheme, or a combination of both (which seems most likely) the Vikings stunning and sudden struggles against the run cannot be glossed over. Bo Mitchell is the VP of Content at SportsDataYou can follow Bo on Twitter at @Bo_Mitchell