One thing to remember about RB receptions is for some reason the Rams like using Mason more in that role than they did Gurley. Makes no sense but they did. Mason even caught 7 passes in one game this season. So while Cunningham is locked in as the third-down RB I could see Mason catching 2-4 passes this week. He seems like a much stronger lock for receptions as a starter than Gurley was which, of course, makes no sense, but is definitely a positive for Mason this week.
In the last four weeks, the 49ers have allowed 7 rushing TDs and nearly 130 rushing yards per game. On paper, this is a terrific matchup for Mason since the Rams will want to run the ball a lot no matter who the RB is. Mason had several good games last season so he's not a bum. He can play. When you combine the matchup and potential volume I would say Mason's ceiling is extremely high. There's definite downside since he's a backup who hasn't looked good all season but as a Gurley owner I'm starting him with a pretty decent degree of confidence.