In the panthers offense, the TDs seem likely to be low no matter what for him. But the thing that will give him legit RB2 value is if he gets opportunities for a full season. In two games, he's had 39 carries and a 4.4 ypc rate, which includes a matchup with easily one of the toughest run defenses in the league. If you extrapolate that over a full 16 game season assuming his lowest seasonal ypc (4.3, from last season), he'd be on pace for over 1,300 yards. I would gladly take that kind of production from my RB3, especially if you sprinkle in a few TDs.
The last 5 games of last season shows that the guy can still get a good number of yards from scrimmage when he gets the touches. And he is totally capable of triple digit total yardage games in the right matchup. The x-factor is whether Stewart will come back this year. If stewart comes back and plays, Williams' value drops below the flex level for sure.
I think he's a legit RB3 right now. I picked up LeVeon Bell, hoping that he gets into game shape and becomes a comparable RB3 by the time Stewart comes off the PUP list (if he does).