Shields, 30, didn't play last season after the Packers released him in February. Before he was released he had played in just two of his last 23 games with Green Bay because of concussions.
The former Pro Bowl cornerback never returned from the concussion he suffered in the Packers' 2016 season opener at Jacksonville. It was the fourth concussion of his NFL career, and second in nine months after missing four games during the 2015 season.
When healthy, Shields was one of the top cover cornerbacks in the league. He has 23 interceptions (including the playoffs) since entering the league in 2010.
ESPN's Rob Demovsky contributed to this report.