The question for Smith is how he recovers from the major knee injury he suffered last December. Smith has been running well and taking part in individual drills in the off-season workouts, but he's still a long way from being cleared to play contact football. The Lions will likely put him on the Physically Unable to Perform list to start training camp and then evaluate him along with the way.
With rookie Jahvid Best and veteran Maurice Morris on the roster, the Lions don't need to be in a hurry to rush Smith back and they won't. For that reason, it's possible Smith might be inactive for the first six weeks of the regular season. A lot of that might depend on how the rest of the running back position is looking.
Kevin Smith's injury was devastating enough for the Lions to draft RB Jahvid Best. Expect Best to be the feature back. If and when Kevin Smith returns at 100%, Smith will fall into a complimentary role.