While the Minnesota Vikings aren't actively shopping backup running back Alexander Mattison, league sources are saying that the Vikings are listening to offers for him with the regular season approaching. Mattison is one of the best handcuff options in fantasy football if you've already selected Vikings starter Dalvin Cook, but the 24-year-old could become even more attractive to fantasy managers if he's traded to a team that gives him a more substantial role in their backfield. If Mattison is...