The Cowboys lost star running back
The graph above depicts every running back who has rushed the ball 100 or more times since the start of the 2012 season versus their Rushing Net Expected Points totals. As you can see (or perhaps you can't since it's small), Marshawn Lynch leads the pack with over 52 Rushing Net Expected Points, while Adrian Peterson and Jamaal Charles rank second and third, respectively.
On the flip side, you see a massive outlier. That outlier is Darren McFadden.
Over the last three seasons, DMC has accumulated a -80.62 Rushing NEP total, which is over 30 expected points worse than the second-worst rusher, Bernard Pierce. The dot in the plot above who ranks third-to-last, for those wondering (or can't see), is none other than Chris Johnson.
The graph shows 95 different running backs, most clustered together towards the zero mark. The difference between McFadden and Pierce, the two worst rushers, is the same as Pierce to the 61st-ranked running back, Peyton Hillis. That's right, the 95th to 94th back has seen roughly the same difference in expected points added as the 94th back to the 61st-ranked one.
Even when you factor in receiving, McFadden is 23 expected points worse than any other running back in the NFL over this time.
If you want to blame McFadden's poor play on a bad offensive line, be my guest. But know that, in each of the last three seasons, the back in Oakland who was closest to McFadden in attempts has far outperformed him on a per-rush basis. In 2012, Marcel Reece's 0.01 Rushing NEP per rush average was 0.20 expected points better than McFadden's -0.19. In 2013, Rashad Jennings outproduced McFadden by a whopping 0.22 expected points per rush. And last season, Latavius Murray was 0.20 Rushing NEP per attempt better than McFadden.
Considering the best 200-plus attempt rusher last season, Jamaal Charles, saw a Rushing NEP per attempt of 0.11, the differences between McFadden and his Oakland teammates within the metric is staggering.
So no, we shouldn't be optimistic about him in Dallas, even if it's behind the best offensive line in the league. Darren McFadden isn't the back we all have wanted him to be.