I would look towards Reggie Bush and Maurice Jones-Drew for examples of how he'll probably be used. Here are their rookie stats:
Reggie Bush 2006 - 155 carries, 565 rushing yards (3.6 YPC), 88 catches, 742 receiving yards (8.4 YPR)
Maurice Jones-Drew 2006 - 166 carries, 941 rushing yards (5.7 YPC), 46 catches, 436 receiving yards (9.5 YPR)
Bush and MJD both averaged about 10 carries per game as rookies. I think Best will probably carry the ball a little bit more than these guys did because there's no Deuce McAllister or Fred Taylor on the Lions. The other Detroit RBs suck, so Best should be forced to pick up some of the slack. I'll say 13 carries per game. I expect his YPC to be a lot higher than Bush's 3.6, but probably lower than MJD's 5.7. I'll split the difference and say:
208 carries, 956 rushing yards (4.7 YPC)
I think he'll be a big factor in the passing game. I don't think he'll get 88 catches like Bush, but I think he'll get more than MJD's 46. I'll say 25% of the gap between MJD and Bush, which would be about 57 catches. I'll estimate his YPR right in the middle at 9.0. All in all, I expect something like:
208 carries, 956 rushing yards (4.7 YPC), 57 catches, 513 receiving yards (9.0) YPR
Reggie scored 8 TDs as a rookie. MJD scored 15. Since the Lions are still a pretty bad team and since Best probably isn't an ideal goal line back, I'll say 10 TDs. I expect him to get a couple more than Reggie because I think he'll break more big plays.
208 carries, 956 rushing yards (4.7 YPC), 57 catches, 513 receiving yards (9.0) YPR, 10 total TDs
That equates to 263.9 points in a 16 game season, which would put him right on the cusp of the top 10 RBs in most years.