Let’s take a look at what happened.
The first thing that jumps off the page for me are the personnel packages the teams have. The 49ers line up with two backs and a tight end. The Jets meanwhile have four defensive linemen, two linebackers, two cornerbacks, and three safeties.
Against this type of offensive package, a defense typically has either three defensive linemen and four linebackers or four defensive linemen and three linebackers. Five defensive backs in this situation is an awfully light grouping.
Bradley McDougald is essentially playing a linebacker role for the Jets on this snap.
In addition to this, the Jets keep the other two safeties deep.
That gives them a light box of seven players. The 49ers have five offensive linemen, a tight end, and fullback. That adds up to seven blockers. Thus there is a blocker for each Jets defender near the play at the snap.
If the Niners run it either somebody is going to have to win his assignment and make a play, or a cornerback is going to have to help in run support.
It doesn’t happen, however.
At the snap Henry Anderson makes a move inside.
Because the run play is going outside, this leads to him getting sealed.
That’s a pretty big problem because he is assigned to the outside gap between the right tackle and the tight end which is now a gaping hole.
The Jets may have stuck McDouglad into this spot because of injuries at linebacker. It clearly would be better to have a real linbeacker, though, as he gets sealed by the tight end.
Because he is blocked, the gap McDougald is assigned to becomes open. Combine that with the block on Anderson, and you have two gaps open. That’s as big of a hole as an NFL running back will ever get.
This is guaranteed to be a big play unless somebody else can cover, but Neville Hewitt gets pushed around.
That run support from the cornerback position I talked about above didn’t come as Quincy Wilson gets taken out by the fullback.
Meanwhile Alec Ogletree is just too slow to move across the field to catch Mostert in time.
It seems like the Niners understood to which side they needed to run the ball. If this play goes to the left side of the formation, Ogletree is much better equipped to take on a block and McDouglad to chase a play down from the backside.
This play was destined to be a negative for the Jets based on execution up front, but what happened in the back turned it into catastrophe. A lot of the blame has to go to rookie Ashtyn Davis.
Moving back before the snap, the Niners motion a receiver from the backside to the play side prior to the snap.
This gives him a running start as he goes to meet his blocking assignment, Davis. This is one of those subtle aspects of Kyle Shanahan’s play design that makes it a little bit more effective.
For his part, Davis just takes too hard of angle. He runs himself right into the blocker and out of the play.
You just can’t have that from the last line of defense.
From that point it’s a foot race, and nobody is going to catch Mostert.
The Jets weren’t going to catch the 49ers either.