Lotulelei, the 14th overall selection in the 2013 draft, spent most of the past five seasons being overshadowed by Panthers teammate Kawann Short, who made the Pro Bowl in 2015. The person spoke on the condition of anonymity because the signing won't become official until the NFL's business year opens on Wednesday.
If Lotulelei serves as the replacement for traded Marcell Dareus, Williams can continue in his role as an an underrated disruptor with at least five sacks in four of his past six seasons. Williams is unsigned and has also contemplated the possibility of retiring.
Just as valuable as Williams was on the field, it was his leadership in the locker room that helped first-time head coach Sean McDermott reinforce the culture change that had to take place for the Bills to grow into a successful franchise again. By signing Lotulelei, Buffalo's front office fills a glaring hole with a familiar face.
Defensive tackle was one of the Bills' biggest needs entering free agency, with only three players - Adolphus Washington, Marquavius Lewis and Rickey Hatley - under contract at the position for next season.
Lotulelei has recorded 141 tackles and 11.5 sacks in his career.
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.
The overhaul comes despite the Bills coming off a 9-7 season in which they ended a 17-season playoff drought.