Mason (foot) was sidelined for Thursday's practice, Matt Maiocco of NBC Sports Bay Area reports.
Mason is considered day-to-day with a sprained foot. When healthy, he is currently slated to back up Christian McCaffrey and Elijah Mitchell in the 49ers' backfield, while Tyrion Davis-Price should be in the mix as well. His status ahead of Week 1 should become more clear next week when San Francisco releases its initial injury report.