Here is some completely unsolicited advice.
Sell him! Sell him now!!
Mostert will be 28 yo to start the 2020 season. Kyle Shanahan has proven that he can make any running back in his system valuable. There will likely be no main guy, no workhouse back for the foreseeable future. No guy who gets 15 touches or a TD every game - on a consistent bassis.
This post was inspired by a recent tweet from FantasyPros. I realize FantasyPros have to put out content. Further, they use the word “IF” in their proposition but every response to this tweet makes me recognize some people are legitimately expecting him to be a lead back next year.
What did/would you trade to get Mostert? What are you trading him away for? Is anyone keeping him despite my unsolicited advice?