Mostert received a veteran's day off Tuesday, David Lombardi of The Athletic reports.
At the start of Tuesday's practice, Mostert wasn't in uniform, with his only subsequent activity a brief stretching session with a 49ers trainer. With no injury to blame, he should be back on the field in the near future and retake the reins of what may be a three-headed backfield with Tevin Coleman and Jerick McKinnon in 2020.