Not sure if Yards per attempt are the most important way to measure an offense in the rushing attack. Even the author acknowledged that a 2 yard carry on 3rd and 1 is a success, but reduces the overall average. I'm sure there are a ton of ways to break down the run game. For Example running off Left Tackle yielded a 4.4 Yard per Carry average, BUT when the author compares that to the Down and Distance, it was the worst place to run for the Bears last year...
Howard simply did not get rolling like in previous year, and maybe that was due to O-line, number of snaps, predictability, whatever it was, this offense is usually based off a fresh pair of legs coming in and producing results without having to "warm up" with several carries. In my eyes that is why Howard was traded, they need a ball carrier that is able to perform his best when he is fresh. They rotate the RBs to keep them fresh, Howard does not fit that mold....