Detroit signed Bentley on Wednesday and put cornerback Josh Wilson on injured reserve with a knee injury.
The Lions cut Bentley in June after drafting him in the third round in 2012. He started eight games and played in 18 games over three seasons with the Lions.
Wilson was injured in Sunday's win at Green Bay, where he started fourth game this season and the 89th of his nine-year career. He signed with the Lions as a free agent last April after playing for Atlanta, Washington, Baltimore and Seattle.
The Lions' secondary took a hit last week when cornerback Rashean Mathis was put on injured reserve because of a concussion.