Williams played to negative reviews in Minnesota last season. He has 85 starts in seven seasons and 12 interceptions, including one over the last two seasons.
"The reality was he was never the same player after suffering a severe neck injury during his first training camp with the Vikings [in 2008]," Vikings beat reporter Judd Zulgad wrote following Williams' release from the team.
Off the field, Williams was the Walter Payton NFL Man of the Year in 2010. On the field, Williams drew skeptical reviews even from his own head coach, Leslie Frazier.
"[He would] like to get more hands on balls, and he'd like to get more turnovers, like to be a little better tackler in certain situations," Frazier told reporters in March. "Those are all things that I'm sure he'd say he wants to improve on, and he will. He still has a chance to. But I still think that neck injury has kind of stuck with him a little bit. But I've seen him improve in some areas where he was good at in the past -- even this past season in certain instances. So, I think he's getting better at some things, but this offseason will be critical to his future success."
There was no real offseason in the NFL this year. Williams' high base salary made him vulnerable to release. The 49ers would be better off at free safety with the younger and more physically imposing Dashon Goldson in the role, but Goldson remains unsigned.
Williams, 29, started 13 games last season.