I'm gonna ignore the rookie year, because... rookies.
ypc - yards per carry, ypr - yards per reception, ypt - yards per touch
2011: 5.2 ypc / 6.9 ypr / 5.7 ypt
2012: 6.0 ypc /10.7 ypr / 6.8 ypt
2013: 4.6 ypc / 5.6 ypr / 4.8 ypt
2014: 3.5 ypc / 8.2 ypr / 4.3 ypt
He improved every year under Chan. Whether that be him learning, or Chan figuring out how to use him, or a combination of the two, that's indisputable (his rookie year was worse than 2011). As soon as Hackett takes over? He regresses majorly. To be fair, he was also injured a bit, and the line made some changes. His carries stayed about the same (202 vs 207) but the amount of receptions dropped dramatically (33 vs 43) and the types of routes changed as well.
This year, Hackett has designed some throws to CJ, already, so that's improving, but we're still power running him up the gut in heavy formations it seems. I'm not really sure what the answer is there, as I'm not an expert in run block design. But the holes certainly aren't there like they were in previous years. Fred can take a small hole, hit it with power, and fall forward for a few yards. CJ, however, cannot. The run designs that Hackett implements is best suited for power running backs. CJ is clearly effective, but on the right team, in the right system.
CJ had a really high yards after contact number in 2012, because the holes were so large that linemen were only getting a hand/arm on him. This year, and last, the holes are smaller, and he's getting swallowed up, or he bounces away from them.
We need a better blocking scheme, or we need to change to a full scale power running attack.