He hasn't been the most durable guy in the past. Do I expect him to play 16? No.
On the other hand he has been very productive when he does play. Last year in the final 6 games he contributed low end RB1 numbers. The Lions offense is extremely RB friendly (you saw it with Best as well). Smith is also a very adept pass catcher and similar to the Eagles and Pats they use little dump offs/screens a lot.
As far as RBs available in that 60-75 ADP he offers as much value/upside as anyone. Only other guys I like as much are Ridley/Mcgahee (in that tier). He will have a big week vs a terrible Rams rush D..
IF Foster is out, should I put in Tate as a starting RB ahead of Sproles? I'd put him flex, but I used it on Miles Austin and Murray is my other RB I played.
It is a .5 PPR league, but also .25 points per carry as well. Screwed up scoring I know.
For future reference, don't use an early player in the flex position. You want to keep that open as long as possible so you have multiple positions to choose a replacement from in case something goes wrong with one of your starters.