Lions' Mathis in concussion protocol

Detroit Lions cornerback Rashean Mathis has entered concussion protocol, putting his status for Sunday in question.

Mathis was evaluated for a concussion late in a Week 7 loss to the Minnesota Vikings, and was then ruled out of their following game against the Kansas City Chiefs with an illness.

The Lions explained Wednesday that Mathis passed the initial tests, but began experiencing headaches on Thursday after the Vikings game and again on the day of the Chiefs game. The team didn't realize the headaches could be concussion-related until days later.

Head coach Jim Caldwell called it "a complicated issue" and said he's unsure of Mathis' game status moving forward.