From the AP via the Seattle Times…

Westbrook, an All-Pro last year, has struggled to put up similar numbers this season. He ranks 32nd in the NFL with 655 yards from scrimmage after finishing first in that category in 2007.

One reason for Westbrook’s declining production may be the holes he used to slither through aren’t quite as big. The loss of two-time Pro Bowl right guard Shawn Andrews (back injury) and poor blocking by inexperienced fullbacks has contributed to the ineffectiveness of the team’s rushing attack.

And…

“Unfortunately, I had a couple injuries here and there,” he said. “I’m trying to get completely healthy and trying to help this team as much as possible. It’s a little bit different because Don (quarterback Donovan McNabb) was hurt a little bit last year, so we ran the ball a little bit more. As a team, we have to be able to run the ball. We still have to be able to establish the run throughout the course of a game. When we have it established, we have to be able to call upon it and effectively use it. We haven’t been able to do that.” (Source)

Could it be a simple as being a little long in the tooth?  He is 29 and has a lot of milage.