Lotulelei, 28, is a five-year vet of the Carolina Panthers. The person spoke on the condition of anonymity because the signing won't become official until the NFL's business year opens on Wednesday.
The signing will help the Bills along the defensive front which suffered greatly after they traded Marcell Dareus to the Jacksonville Jaguars last season. Williams is unsigned and has also contemplated the possibility of retiring.
Bills coach Sean McDermott was Carolina's defensive coordinator for the first four seasons of Lotulelei's career. Lotulelei will now provide an anchor for the next few years for a defense being remade in general manager Brandon Beane's image.
Kyle Williams has been one of the Bills' mainstays, but he's an unrestricted free agent and hasn't made it clear if he wants to play another season at age 34.
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Lotulelei has forced two fumbles and has 11 1/2 sacks in 76 career games, and was named to the National Football League all-rookie team in 2013.
The deals continued strengthening Buffalo's bid to consider selecting one of several highly regarded quarterback prospects in the first round of the draft next month.
Williams had a cameo role on offense in the team's season finale at Miami that they had to win to keep their playoff hopes alive when he plunged into the end zone from a yard out after lining up as a fullback.