Rashean Mathis has been feeling the effects of a concussion suffered in Week 7 for nearly three weeks, and the Lions decided the brain injury will end his season.
The team placed the 35-year-old cornerback on injured reserve Saturday, ending his season before Sunday's game against the Green Bay Packers (1 p.m., FOX).
Lions coach Jim Caldwell referred to Mathis' sustained brain injury as "a complicated issue" on Wednesday before head trainer Kevin Bastin made a rare media address to explain it.
According to Bastin, the Lions tested Mathis for a concussion in the fourth quarter of an Oct.