Why exactly? We did it with Bush @2. We also have Kamara who may be in his last year of a high producing. On top of that, a rookie still needs to condition their body for the rigors of the NFL. We always see rookies at all positions play limited roles their rookie year. Why should this be any different?
It's silly to think Jeanty wouldn't be splitting reps if he's drafted by a team with a pro-bowl RB. Saints, Ravens, Eagles... Likely even the Patriots (which is who I think will draft him) he would likely be splitting reps with Stevenson.