I think Leach will mainly be missed in those off-tackle plays - Example:
Foster's long run against Oakland. When Leach blocked a guy, that guy was
out of the play. Casey (or Vickers or whomever) will just have to get sufficiently in the way so Foster can use his skills, which are considerable. I think AF is going to be a marked man this year so the Texans will need to spread the runs around (Ward, Tate) and get creative....