Chris Johnson's statistical woes are mostly a matter of his running behind one of the worst sets of run-blockers in the NFL.
The Titans' run-blockers have given their ball carriers a favorable blocking situation on 29 percent of their rushing attempts this season (a favorable blocking situation being loosely defined as when the blockers don't allow the defenders to do anything to disrupt the rush attempt).
To get an idea of how bad that total is, consider that the Tampa Bay Buccaneers ranked dead last in the league in that metric in 2010 and they managed to post a 33.6 percent rate in this category.
This is no surprise to anyone that has watched Johnson carry the ball this season. It's rare that he has any space to move and often takes his first hit at the line of scrimmage or in the backfield.