While the surgery success rate is good, it's rare to see a running back come back at 100% mentally and physically from a bad knee injury. Adrian Peterson is a rare example of a player who did.
With 2 knee injuries in two years, I do not think Bell will be as good as he was last year. The Steelers openly admitted to more of a two-back attack plan this season, lighting Bell's workload and using a near equally good, and much cheaper Williams more often.
If you ask me, running the ball on our team is easier than most. We have ag good OL, Ben, and great skill players. As such there is more room to run.
I do not want to give any running back franchise type money over 4 years or more, which is what Bell's agent will be asking for when his contract expires.
Very few running backs are worth that type of money, and those who need to prove they can stay healthy and clear of off the field issues.