I'm choosing to wait for an announcement from the Athletic / Football Departments on Devon Johnson. If he does have a season-ending injury, I'm sure they'll announce that; there would be no reason to keep that one a secret (like temporary position injuries, etc.)
As for Foster, I hope (candidly) that he does better at RB than he has at slot receiver; don't know if it is nerves or what, but he's been the least (or near the least) productive receiver on the team so far this year.