In my video reviews for the Daily News the last couple weeks, I've tried to make the point that at this point in the season the success of the Chip Kelly running game depends as much on what the opposing defense does (and is structured to do) as it does on what the Eagles do.
The Detroit Lions have a great run defense this year. Unfortunately for them, the style they play doesn't match up well against what the Eagles like to do.
The Vikings don't really have a great anything, but they committed to -- and had the personnel to execute -- a scheme that could mostly take away the Kelly-run-game-without-a-running-quarterback.
In fact, what we've seen so far this season is that the a priori quality of the opposing run defense isn't particularly predictive of how Shady will fare against them.
As the next chart shows, McCoy's best days running the football (as defined by total yards) have come against defenses that were actually better at stopping the run (as defined by rush yards per attempt after removing McCoy's stats against them):
Conversely, Shady has had better days running against teams with worse passing defenses (those are the orange dots).
Does this small sampling of performance actually prove anything in a hard-core statistical sense? No, not at all. But I think it supports a broader theory of how one defends the Kelly offense.
Even with Foles' lack of mobility, it's really hard to defend the Kelly run game by having an even numbers of defenders and blockers in the box. You really need that seventh man to give yourself a chance. But you also can't really defend the Kelly passing game by playing passive, off coverages that give the Eagles lots of opportunities to kill you with packaged plays and wide receiver screens.
This combination of factors means that committing to a defense to stop the Kelly scheme requires you to have defensive backs who can play aggressively and without much help outside, so you can overload inside and keep just one safety back.
Without that -- without a plus passing defense -- your run defense will be stretched past its breaking point, even if it matches up well against more traditional rushing attacks.