Cox, who has a back injury, and Mathis, who has a groin injury, were limited in practice on Wednesday.
Also not at practice for the Jaguars on Thursday were full back Greg Jones, defensive back Dwight Lowery, center Brad Meester and running back Maurice Jones-Drew, who will sit out Sunday with a sprained foot.
Quarterback Blaine Gabbert, who has been battling a left shoulder injury the past couple weeks, was a full participant Thursday. Wide receiver Lauren Robinson, who is coming off a concussion, also practiced fully.
Running back Montell Owens (shoulder) was limited in practice Thursday. He did not participate Wednesday.
The biggest name on the list is wide receiver Calvin Johnson, who has a knee injury and did not practice. Defensive end Cliff Avril, safety Louis Delmas and safety Amari Spievey also did not practice.
The Jaguars (1-6) host the Lions (3-4) on Sunday at 1 p.m. ET.