This is yet another tough break for Mostert, who missed half of last season with an ankle injury. Now 29 years old, "Must Start" looks like a player whose body is just giving out, this time it's with chipped cartilage in his knee. With Mostert out, the 49ers will turn to some combination of Elijah Mitchell, JaMycal Hasty and Trey Sermon, who was inactive in the first game. This is going to be a tough backfield to predict and Kyle Shanahan is being very cryptic as to what to expect. At this point, all three of those running backs need to be owned as things play out. As for Mostert, he's likely worth holding on to at the moment, but his career seems to be running on fumes.