There’s no rule about how one should fill the flex position, either with a running back or a wide receiver. It changes each week depending on top options and matchups, and this week, plenty of tight ends are attractive choices as well. The point is, on your team, it could be any of the three positions.
It’s worth noting that a mere four of the top 10 players in these -- the Week 5 flex rankings -- play running back. I cannot recall a time when the position featured less depth (and by the way, that’s even at the top). How many guys can we truly trust? Not many when compared to wide receiver, that’s for sure.
Good luck with lineups this week, the first with four teams assuming a bye week. It’s not like the absent teams were superstar-laden anyway, with the teams that played in London last week plus Cam Newton and Adrian Peterson sitting. So enjoy this week. It only gets worse in terms of byes, and certainly as the fall continues, there will be more injuries. It seems like everyone is limited at practice this week. Don’t limit your decisions in Week 5!