I have to think it would almost be unprecedented for a team with World Series aspirations to yank a good-but-not-great player out of the lineup and hand the position to an untested rookie in mid-season. No matter how promising a rookie is, it's pretty unexpected for one to be better than an average player.
Look at the 2011 top position prospects: Trout, Harper, Brown, Hosmer, Moustakas, Ackley, Machado, Freeman, Jennings, Belt.
Some of them have struggled (even Trout his first season of 140 PA), some have held their own. Only Harper, in my opinion, came up his first season and had the kind of WOW performance I'd expect to push an above-average player out of a key position.
But I don't think it's reasonable to expect Rendon to be capable of a top-3 ROY type performance without having played more than 65 games in a year, in his life.