Mason (ankle) has been ruled out for Sunday's game against the Bears.
In the wake of the 49ers' Week 13 loss at Buffalo, the 49ers lost their top two running backs for the foreseeable future: Christian McCaffrey suffered a PCL injury in his right knee and was placed on injured reserve Tuesday, while Mason was expected to join him on IR due to a high-ankle sprain. The transaction for Mason has yet to be made but likely will happen by Saturday afternoon. With both players out Sunday, Isaac Guerendo is in line to lead San Francisco's backfield, while Patrick Taylor, Israel Abanikanda and practice-squad member Ke'Shawn Vaughn are the candidates to fill in behind him.