Ryan, who was named head coach on Monday, announced the hire of close friend Dennis Thurman as defensive coordinator Thursday, along with seven more coaches who come along to the Bills from his New York Jets staff.
Thurman served in the same role under Ryan with the Jets from 2013-14 after coaching the defensive backs from 2009-12.
The two have worked together since being members of the Baltimore Ravens' defensive staff in 2002.
Over the last two seasons, the Jets' defense has held opposing rushers to the second fewest rushing yards per game, allowing only 90.7 yards per game. They led the NFL by only allowing 3.56 yards per rush over the past two seasons.
In 2014, Buffalo's defense finished fourth in the NFL in yards allowed and led the league with 54 sacks. Thurman replaces Jim Schwartz, whose contract was terminated, allowing him to pursue other jobs.
Before joining the Jets, Thurman was part of the Ravens' coaching staff for six years from 2002-07. He spent his first two seasons as a defensive assistant and his last four seasons coaching the secondary.
Bobby April III (linebackers), Karl Dunbar (defensive line), Tim McDonald (defensive backs), Jason Oszvart (strength and conditioning assistant), Eric Smith (special teams assistant), Tony Sparano Jr. (tight ends) and Jeff Weeks (assistant defensive line) were also hired by the Bills on Thursday.