Football Outsiders has Chris Johnson ranked as the least valuable running back in the NFL on Sunday:

Johnson's best run on Sunday was an 11-yard gain on first-and-10. His other 13 carries gained only 23 yards, and he was stuffed for no gain or a loss five times. This was against the Colts, a defense that came into the game giving up 4.6 yards per run. He was equally impotent as a receiver, catching three-of-five passes for 17 yards, with his longest gain a 10-yard catch on third-and-19. For the season, Johnson is dead last in FO's rankings for total rushing value, and he also has negative value as a receiver. By the end of the game Sunday, Johnson had been essentially benched for Javon Ringer. If the Titans are smart, they'll make that switch permanent for the rest of the year.

Mike Munchak said again yesterday that Chris Johnson is still the starter, but if he doesn't kick it into gear soon it will be Johnson spelling Ringer.  That would be a tough call for a first year coach to make, but in the end he has to do what is best for his football team.  This is going to be an interesting to watch.