The initial prognosis for rookie running back Ryan Williams' knee injury is a patellar tendon tear, which means he'll almost certainly miss the rest of the season. For a little more detail into the injury, I consulted SportsMD.com (tip of the cap to Kent Somers for the link) and there is some generally good news, though time is not on Williams' side.
Per SportsMD.com:
Surgical repair, while not an emergency, should be performed soon after the injury occurred. This allows for an easier repair before a significant accumulation of scar tissue. Most surgeons perform a direct repair of the tendon if it is torn in the middle. If the tendon is torn off of the bone, several different techniques can be utilized such as reattaching the tendon to the bone with a combination of sutures and bone tunnels. Regardless of technique, the goal of surgical repair is to re-approximate the tendon ends in a secure fashion to allow it to heal.
The good news is that if this is the route they take with Williams' injury, in most cases full recovery and retention of power and strength can be expected.
If the repair is done acutely (within 3 weeks), most athletes report good or excellent results. Most athletes are able to return to their previous level of competition and do not report residual weakness or discomfort.
One caveat is that as with any injury, there is greater risk or re-injury, loss of mobility/strength, and infection.
Loss of knee motion and quadriceps weakness are the two most common problems after surgery. Whether this is due to the initial injury or the surgical repair is difficult to determine, but likely due to the initial injury. With postoperative physical therapy as discussed above, the goal is to minimize these problems. Less common complications include persistent knee effusion, infection, and re-rupture of the repaired tendon.
This is hardly comforting to Arizona fans at the moment, but there are some serviceable free agent running backs still out there. Clinton Portis, Correll Buckhalter, Brian Westbrook, Laurence Maroney, LenDale White, Dominic Rhodes, Mike Hart ,Chad Simpson, DeShawn Wynn, Kenneth Darby, and Quinton Ganther, to name a few.