If I am the Dolphins front office and Foster is healthy they have to make the move to sign him. You don't think twice about this and you make this move. Is Foster and every down #1 RB anymore? NO! Clearly he isn't and he can't hold up to that type of pounding. But guess what, Jay Ajayi isn't a #1 RB either. Ajayi's knee is bone on bone, we all knew that when we drafted him, and then he spent most of last year injured with various injuries.
Other Dolphins running backs on the roster like Kenyan Drake also had a spotty injury history while in college and he is already dinged up and training camp hasn't even started yet. Miami needs a proven vet back on this team. Again, this isn't the Foster of old. He isn't going for 1500 yards and 13 TD's. But he is a guy who can pick up a blitz in the backfield (something Ajayi and Williams struggle with and with Drake as a rookie is probably not going to do well with) and he is a guy who can take the load off of Jay Ajayi and not have too much pressure on him.
If healthy, make that signing Miami!