In both Week 1 and Week 2 the Detroit Lions ceded long runs that seemed to put the nail in the coffin. Against the Jets, a 62-yard scamper capped a nightmarish third quarter, pushing the Lions’ deficit to 31 points. Last week against the 49ers, Matt Breida’s 66-yard touchdown made it a two-score game as the game entered the fourth quarter. Though the Lions were able to work their way back into the game, that was a pivotal play that ended up being extremely costly.

Unfortunately, the Lions’ run defense problems run deeper than those two plays. Take them away statistically, and Detroit is still ceding 3.7 a carry. Keep those two plays in and that number balloons to 5.6. The Lions have allowed the most carries of 20+ yards (4) and the rest of the league combined has only ceded three rushes of over 40 yards, while the Lions are responsible for two on their own.

So what the heck is going wrong here? Let’s go to the tape and see what ailed this defense on Sunday against the 49ers.

Two-gap defense

Before we get into the film, it’s important to note the Lions’ drastic change in defensive scheme. While the Lions aren’t strictly a two-gap defense, it is their bread and butter. Remember that part of the reason Haloti Ngata turned down a deal in Detroit is because he preferred to one-gap in Philly.

So what is a two-gap defense? Well, just like the name suggests, defensive linemen in this scheme are responsible for covering two gaps along the line. This requires big, bulky lineman who prioritize holding their ground over knifing through the offensive line. Strength and size are preferred to speed and agility. The point is to eat up offensive linemen, keep your eyes on the backfield, and clog the appropriate hole when a running back makes his cut.

Here’s what it looks like:

It’s not an easy task for the defensive linemen, who not only have to be patient, but have to be strong and physical. But there’s a reason Matt Patricia preferred guys like Sylvester Williams (329 pounds) in free agency. Here’s what went wrong on the play above:

Let’s just focus on Ricky Jean Francois, because this is the moment of truth. Francois has done a good job holding his ground and properly has his eyes on the backfield. Breida shows like he’s going to kick it inside before cutting outside. Francois sees his hips squared to the middle, so instead of keeping his outside leverage, he tries to push through the offensive lineman, clogging up the interior. That leaves a gap “small” enough for Breida to slip through.

Now another important aspect of the two-gap system is linebacker play. With bulky, patient linemen sometimes occupying multiple offensive linemen, it frees up linebackers to pursue aggressively and often unblocked. It’s up to them to read correctly and attack. Unfortunately, on this play, the Lions don’t get the necessary reads from the second level to find success.

Early on in the play, the 49ers are showing a fake end around. Look at all the players in front of circled linebacker Jarrad Davis. They are all blocking downfield to the left. Matt Breida is also running left, before he eventually counters to the right. This movement fools Davis into sliding ever so slightly to the inside. That allows 49ers tight end George Kittle (No. 85 on the right) to take him completely out of the play.

Davis has slid so far inside (note the hashmark) that not only is he an easy block for the tight end, but he has now impeded the free linebacker (Christian Jones) from any path to the running back. Put all of it together, and what do you get?

Pain on the very first defensive play of the game.

This wasn’t an isolated incident, and you can see the same sort of issues on the 49ers’ other big runs on Sunday:

On the second day of Christmas, the Lions gave to me... pic.twitter.com/K8vlWhkPa5 — Matt Waldman (@MattWaldman) September 17, 2018

On this play, nicely broken down by Matt Waldman, Romeo Okwara and Jarrad Davis are the key players. Okwara, like Francois, does a good job of holding ground and being patient. But he’s unable to shed the block at the last moment.

Davis, the linebacker freed up thanks to A’Shawn Robinson nicely occupying two linemen, hesitates for just a second, expecting a cutback from Breida. But Robinson has essentially shoved the center into that gap, so there’s no reason for Davis to cover the cutback. That slight hesitation is all Breida needed to hit the hole hard, and Davis’ arm tackle isn’t good enough to take him down.

To Breida’s credit, his speed (4.38 40-yard dash, 9.48 RAS) and decisive running style were the perfect combination to beat a linebacking crew that is still trying to figure things out and a defensive line that isn’t quite athletic enough to consistently win on stretch plays. But the Lions are going to face a lot of talented running backs this year, so they can’t afford these kind of mistakes any longer.