The Titans will be a run-first team this year, but that doesn't make their starting running back their most valuable asset.
In fact, I would argue that it is better value to draft Shonn Greene in the middle rounds, rather than to take Chris Johnson in the top 10-12 overall. Here is why:
Chris Johnson is not the guy that the Titans want running between the tackles in the red zone. While Chris Johnson may get 10-15 more carries than Shonn Greene throughout the course of the game, it is important to remember that in a standard league a rushing touchdown is worth six points.
So lets say Chris Johnson begins the drive from the 20 as the starting running back, he would have to get the Titans to the other 20 yard line on his own to equate to a single, one-yard touch down dive from Shonn Greene.
Now, that isn't to say that Shonn Greene is just "money" from the red zone, but it is a struggle to imagine a bruiser-type running back not being able to put it in the endzone if he has Chance Warmack and Andy Levitre as the starting guards.
At the end of the season I do expect Chris Johnson to edge out Shonn Greene in fantasy points, but he has been far from consistent in his time in the NFL. The value of taking Aaron Rodgers, Calvin Johnson, or another top-tier running back in the top ten is far better than to take Chris Johnson, and taking his goal-line backup is a smart move.