Williams' last game saw him sustain head and neck injuries on an illegal blind-side block by Miami Dolphins wide receiver Jarvis Landry, who was fined by the NFL for the hit. Landry later apologized for the block in that game, a 28-25 win by the Dolphins on Oct. 23, 2016.
The 27-year-old Williams sustained a neck injury in a game against the New England Patriots in September 2015 that saw him leave the field in an ambulance and eventually end his season. He also suffered a concussion in practice during the 2016 training camp.
"They all told me that because of my history of concussions and neck issues, their doctors wouldn't clear me to play," Williams wrote Wednesday.
Williams also apologized to Bills fans for a rocky start to his time in Buffalo in his letter on Wednesday.
"So when I woke up on January 1, 2018, and I hadn't signed with a team, and I knew it was over, I just started to kind of wonder: What else could I love like that?" Williams wrote. "What else in this world is gonna give me that fix -- that adrenaline, that anger, that feeling of dominating somebody to the point where they don't want any piece of you. What else is there? Honestly, I haven't found the answer yet. It's all just still so new.
"All I know is, I'm never going to play another down in the NFL. And that hurts, for a lot of reasons. But when I think about those first two years in Buffalo -- those wasted years when I was just a young punk who didn't get it -- man ... it hurts a lot more."
Williams recorded 257 tackles and seven interceptions in 59 career contests since being selected by Buffalo with a second-round pick in the 2011 draft.