Carp is coming off of an extended layoff. We should expect nagging stuff like feet, ankles, etc. until his legs get into game shape. This is probably nothing more than a minor strain that the FO is just resting so it doesn't get worse.
Thing is, with this much young talent on our OL, it's very possible that the coaching staff could just get impatient, or that we need the roster spot too badly, or Carp just gets pushed out by a younger, healthier player. He's got a ton of potential, but you can't realize potential on the sidelines.