Since when does 7 games define your career?
I dont think it is fair to say that RB is such an easy position that players should not require any time to adjust to the position in the NFL. Maybe if they are straight ahead bruising runners or come into a great readymade system or a system similar to their college team that is true. I think it depends on their particular skills and the situation. Supposing that Reggie Bush's best skill is his escapability and that NFL defenders are faster, maybe it will take time for him to adjust his reaction time to them and escape effectively. Reggie Bush does not just run straight ahead, he thinks, constantly makes decisions of which direction to go with his top speed and maneuverability. The defenders are faster now so he has to react faster. That could take time, but qbs, linebackers, running backs, etc all have to do it and some improve even after 7 whole games.
Also, Bush is not a midget. In theory if he cannot get faster he could try to get bigger without getting slower. I would not recommend it at this point but it is an option if his career has setbacks and he wants to try a different way of being a feature back. Bush is listed at 6'0 and all finesse backs we are comparing him to are 5'8-5'10, though I do not know if their measurements are more accurate.
Maybe Bush would be picked a little lower. Probably not too much lower though because he showed a lot of potential in college and can be a very exciting player. But remember all the hype on another USC talent the year before, Mike Williams. The Lions took him #10 in the first round and many said he would be great. He had the size, hands, and big play ability if not the speed. However, he was a bust. Now imagine everything Mike Williams was every hyped to be and everything his biggest fans thought he could possibly do in his rookie season. Then subtract attitude and add speed. This is Marques Colston. If the draft were repeated today Marques Colston would be selected no later than by SF at #6 overall. I think Reggie Bush would still go somewhere in the top 5. Who knows. The Texans might take a QB. It would be a very different draft. We might be tempted to look at Ferguson, Hawk, or Colston instead, but I am happy that the draft was in April because we got Bush and Colston.