Head coach Jim Caldwell said Wednesday that Zenner remains in the league's concussion protocol, Tim Twentyman of Lions.com reports.
Zenner suffered the head injury in last week's victory over the Bears, and he is apparently still dealing with symptoms three days later. The Lions' running back situation for Week 15 is murky at the moment with both Zenner and Theo Riddick (wrist) less than 100 percent, but it should become clearer based on both players' participation, or lack thereof, in practice over the next couple days.