Ihedigbo played two seasons, 2009 and 2010, for the New York Jets when Ryan served as head coach. He played in 29 games over that span, all as a reserve.
The Bills have been searching for a starting safety opposite Corey Graham since Aaron Williams was placed on injured reserve Nov. 1. Veteran backup Robert Blanton has started the past two games in Williams' place.
The Bills released safety Duke Williams in a corresponding move.
The 32-year-old Ihedigbo spent the past two seasons with the Lions, the first season-and-a-half as the team's starting strong safety.
He and Glover Quin were considered one of the best safety tandems in the league in 2014, but Ihedigbo was replaced in the lineup by Isa Abdul-Quddus midway through the 2015 season. It was a season Ihedigbo called "humbling," and he told ESPN "without a doubt" he believes he can still be a NFL starter.
By the end of the 2015 season, Ihedigbo was playing on some special-teams units -- the same role he had when he started his career as an undrafted free agent with the Jets.
Ihedigbo has played with the Jets, Patriots, Ravens and Lions and won a Super Bowl in Baltimore. In his career, he has made 332 tackles with nine sacks, eight interceptions, nine forced fumbles and three fumble recoveries.
ESPN's Mike Rodak and Michael Rothstein contributed to this report.