Bills general manager Doug Whaley said in a statement:
We want to thank Boobie, Kraig and Mario for everything they did during their years with our franchise. Each of them contributed to our team in a unique way. Days like this are never easy and this is a part of the business that is difficult on everyone. We will now focus our attention on moving forward as we continue to prepare for free agency, the upcoming Draft and the 2016 season.
Williams' tenure in Buffalo appeared to be coming to an end as the Bills were finishing a disappointing 2015 season.
This was Williams' worst season with the Bills, as he recorded just five sacks after having at least 10.5 each season from 2012 to 2014, and his declining performance in Ryan's system couldn't support a salary that included $36.4 million in total cap hits over the next two years, according to Spotrac.
The Bills will get $12.9 million in 2016 cap space by cutting Williams, who has a $7 million dead cap hit for next season, so obtaining financial flexibility and getting rid of a player who clearly wasn't happy makes this a win-win for the franchise.
NFL Network's Albert Breer (via Conor Orr of NFL.com) reported another major move, which the Bills confirmed, coupled with Williams' release: Left tackle Cordy Glenn was given the non-exclusive franchise tag and "is set to make $13,706,000 per the official franchise tag numbers for 2016."
Rapoport reported on Dec. 20 (via Orr) that the former No. 1 overall pick wasn't happy with Buffalo head coach Rex Ryan's defensive scheme and that it wouldn't be a surprise if Williams didn't return to the team next season.
John Wawrow of the Associated Press later reported prior to Week 17 that the Bills had already made the decision to cut Williams during the offseason:
A person with direct knowledge of discussions has told The Associated Press that the Buffalo Bills plan to cut high-priced defensive end Mario Williams this offseason.
The person spoke Thursday on the condition of anonymity because the team does not publicly discuss its player personnel plans.
Williams seemed to seal his fate following a Week 15 loss against Washington in which the defense gave up 431 total yards and four touchdown passes to Kirk Cousins, telling reporters he didn't see the point in taking a pay cut, via ESPN's Mike Rodak:
If this is the turnout of our defense, how does [a pay cut] even sound right? Like I said, you can change. People are scared of change and so forth. I'm all for it, because at the end of the day, if it works, so be it. Because I don't need to sit here and say: 'Yeah, I wish Jerry [Hughes] had 15 sacks. I wish Marcell [Dareus] had 15 sacks. I wish I had 15 sacks,' across the board.
Williams will be just 31 years old next season and is not far removed from being an excellent pass-rusher in a league that is always looking for that type of player. Now he has been given an opportunity to find a coaching staff and scheme that suits his skill set and to revitalize a career that hit a low point in 2015.