Westbrook might have a low duribility rating. I mean like the person above me said its just like real life. Especially at the RB position, RB take a beating in the NFL. Much like the game. Every carry, every hit taken, every blitz pick up that they pick up, any contact at all will effect a players health rating. The key is like the person above me is cancel practice a day, and he should regain some health. its worked out for me with the Dolphins.
As far as the potential going down...that might be a few things.
1. No production, or poor producton.
2. Might not be a starter
3. there role on the team, and during games.
I signed Nnamdi Asomugha( the Raiders great CB) from the preseason with the Dolphins. Im now in my 2nd year.(5-1 baby, with rookie QB Dane Morrow) But the first couple games he had struggled. He went from a 96( man coverage philosophy) to a 92, then back up to 93, and finally he has been playing better, he is at a 94 now. It happens. This last game .vs. the Jags, he returned to INT's for TD. At the end of the game his overall and potential went from a 92, to a 94. Its all about production, & contribution to the team.