Rob and Rex Ryan are working together in an official capacity. According to Fox Sports' Alex Marvez (and later confirmed on the team's Twitter page), Rob is joining the Buffalo Bills' defensive staff as an assistant coach, working under his twin brother Rex.
Rob last served as the defensive coordinator of the New Orleans Saints, working with Sean Payton since 2013. He was fired in the middle of the 2015 season, during which the Saints' defense finished 31st in both rushing and passing yards allowed per game. Rob also had defensive coordinator stints with the Dallas Cowboys, Cleveland Browns and Oakland Raiders.
The Bills hired Rex before the 2015 season, hoping that he would deliver the team its first playoff appearance since 1999. Instead, Buffalo slumped to an 8-8 record, extending the longest current playoff drought in the NFL. A large part of the Bills' disappointing finish was an underachieving defense, which was 19th in yards allowed per game just one year after ranking fourth in the same category. The Ryan brothers are joining forces to try to turn this unit around.