as to the prognosis, as was noted upthread, this is the injury clayton suffered around game four or five last year... he wasn't recovered enough for rams to sign him (who reportedly wanted to if healthy), but he was recently reported to be working out in NE (not sure if he had a physical?)... caddy was also mentioned... he had two (different knees)... he didn't really look the same after two surgeries, but i didn't remember after the first one... seemed like he had regained some of his burst... article excerpt* below i think mentions he started the following season (post-FIRST injury) on PUP, but returned strong, before second injury...
i was really looking forward to seeing williams... that is having beanie in two leagues and williams in none... actual football always takes primacy over fantasy implications in my world, and i always hate to see players injured... the little i saw of williams highlights, he looked like a special RB, with possible top 5-10 upside... in some ways he looked like deangelo, maybe stronger, deangelos cuts were a little more looping and elliptical, with williams a bit more direct in his angles... there was one play where it looked like he jumped in from past the 5 yard line, and landed a few yards in the end zone, and was in the air for close to 20 feet... i think he is (hopefully not was) a rare talent... hope for a speedy, as close as possible to complete recovery...
*
http://sports.espn.go.com/nfl/news/story?id=3798969
"...The 2005 Offensive Rookie of the Year when he rushed for 1,178 yards and six touchdowns, Williams' production has been limited the past three seasons because of injuries. He missed the final 12 games of 2007 after undergoing surgery on his right knee and sat out the first 10 games of this year.
On Sunday, he was having his best game since being activated from the physically unable to perform list in November. In addition to scoring two touchdowns and rushing for 78 yards on 12 carries, he also had a team-high five receptions for 37 yards against the Raiders..."
** ortho site on patellar tendon...
http://orthoinfo.aaos.org/topic.cfm?topic=a00512