Miami let Reggie Bush sign with Detroit because they are excited about Lamar Miller's future based on his 4.9 yards per carry average, and upside. Miller's teammates sing his praises regularly. The Dolphins are concerned about Daniel Thomas' durability, and his ability to be the big, physical goal line runner this offense needs. The Dolphins will likely add another young back, or two via the draft.
We're expecting Lamar Miller to serve as Miami's RB1, though as the article mentions, the Dolphins are expected to add a RB or two this offseason.