The running back is heading into the final season of his rookie deal with the AFC West side, having been a first-round draft pick in 2008.
The team's head coach Dennis Allen told the league's website: "Obviously, he never felt comfortable in what we were doing last year.
"That led to some questions about him, but if you watch practice, the guy's still explosive. He's still a threat to take it all the way. And he feels more comfortable. If you don't quite trust it, there's always that doubt."
The 25-year-old has missed 13 games in the last two seasons with injury.