While the main keys to the Eagles loss to the Steelers were the three fumbles by Michael Vick, two of which were lost, there's not much film study that can be done to analyze ball security.  So this week we're taking a look at the third-and-12 conversion the Steelers made on their final drive, which led to the game-winning field goal.

It was a blown coverage by the Eagles, and a play the Steelers had planned to come back to after running it earlier in the game.  Pittsburgh quarterback Ben Roethlisberger mentioned that the Steelers had run the same play against the same coverage, but he was under pressure and had to dump the ball off to his tight end, Heath Miller.

Big Ben saw Antonio Brown open, though, and knew what to look for when they ran the same play a second time.  Here's what Roethlisberger saw on the key play.

Here's a look at the pre-snap alignment. The Steelers are in a three-wide shotgun formation, and Roethlisberger has a tight end to his right and a running back to his left. Emmanuel Sanders is lined up on the outside on the right, against Nnamdi Asomugha. Antonio Brown is in the slot across from rookie cornerback Brandon Boykin.  The Eagles are in their nickel defense.

The Steelers brought Sanders in motion, moving him into the slot. The receivers are now near each other, which is going to be critical for the play, as their two routes play off of each other, making it difficult to maintain tight coverage - almost like a pick play.  The Eagles also did not follow Sanders with Asomugha, which is a partial sign that they are in zone coverage. It's not a dead giveaway, as the Eagles have not been following receivers this year anyway, and nobody moved all the way across the formation.

Here you can see the Eagles dropping into their zone coverages.  Notice that DeMeco Ryans is dropping deep down the middle of the field, as the Eagles are in a Tampa-2 zone. The difference between the Tampa-2 and a standard Cover-2 is the deep drop by the middle linebacker, which means the other two intermediate zones must pinch toward the middle a little bit more to close that hole in the zones.

Sanders route takes him down the seam, in part ensuring that Ryans and safety Nate Allen both stay deep and cannot come up to help cover the middle of the field. He also provides a bit of a pick for Brown, as they cross near each other.

Brown's job is to settle into the middle of the defense and find an opening in the zone coverage. Look back into the pocket, and you'll see that Roethlisberger is about to be under duress.

Ben Roethlisberger comes under pressure from Eagles defensive end Jason Babin in the pocket, but Babin cannot bring the big quarterback down, only getting a hand on his leg as he goes by. Roethlisberger has room to step up in the pocket, while continuing to look downfield.

As the play further develops, the gap in the middle of the Eagles defense becomes apparent. Boykin does not drop deep enough and loses Brown a bit over the middle of the field, while Ryans and Allen are taken downfield by Sanders. Asomugha stays in his short zone and can't drop back much farther, or Miller will have room to rumble for a first down even if he catches the pass underneath.

The responsibility here falls on Boykin, who needs to stay with Brown and avoid losing him over the middle.  Boykin's position would be somewhat acceptable if this were a traditional Cover-2, but in a Tampa-2, he's giving up a big gain.

Here's the final look at the play. Roethlisberger has stepped up in the pocket and found a huge passing lane to Brown, who Boykin has now lost by a few steps. The pass is about to reach Brown, who is already beyond the first down markers and will turn it upfield for a 20-yard gain.

In conclusion, the fault on this play lies on Brandon Boykin, for losing his man in coverage over the middle of the field.

A special thank you to Jimmy Kemspki of Blogging the Beast for sending over some of the quotes from Nnamdi Asomugha used to aid in the analysis of the play.

Ryan Messick covers the Eagles for 97.3 ESPN FM.  Follow him on Twitter.