William Jackson III
A free agent who hasn't played with the Buffalo Bills but could be familiar with the defensive scheme that Sean McDermott runs is William Jackson III. He began his career with the Cincinnati Bengals, playing three seasons there, before signing as a free agent with Washington in 2021. He played the last two seasons with the Commanders but didn't finish last season in Washington as he was traded to the Pittsburgh Steelers.
The reason he could be somewhat familiar with this defense is he played under Ron Rivera in that season and a half in Washington. McDermott and Rivera both run similar defenses with McDermott being a long-time defensive coordinator under Rivera when both were with the Carolina Panthers.
The problem is that Jackson wasn't necessarily very good during his time in Washington. He allowed eight receiving touchdowns in 16 games. Last season, quarterbacks throwing in his direction were completing over 70% of their passes.
However, not all situations are equal and Washington's defense doesn't have the same talent that Buffalo does. It wouldn't be surprising to see the team bring him in for a workout to evaluate and see if he could be a good fit.