The Titans currently have a backfield full of complementary guys, but not a single guy that can really be a feature running back.
Since running backs don't hold nearly as much value as the once did in the draft, this need doesn't have to be addressed in the first round. The Titans can easily end up finding their guy with their second- or fourth-round picks.
Tre Mason is a popular choice right now and could maybe be a possibility in the second round. He could come in and solidify the Titans' rushing attack, and free up newly acquired Dexter McCluster to be used in other creative ways.
With the departure of Chris Johnson, the Titans need to try to find a guy that can be depended on to carry the load for years to come. These second-tier guys like Shonn Greene and McCluster are nice options, but they're not guys that can be relied upon as consistent grinders during a full 16-game season.
Also, McCluster is more of an all-around player on offense. He's not meant to be the top guy at running back. You can't have your top running back lining up in the slot and returning kicks on special teams if he's meant to be your main choice at running back.
Furthermore, Greene is purely a 10-15 carries-per-game type of guy. There's also health concerns with Greene, leaving the Titans in need of one more piece that can really lead the charge in the near future.
The Titans need to quickly forget about Johnson, and go find another rising star in the draft.