I don't know a whole lot about how developing prospects work, but I went back and looked at the AAA stats of Ellsbury, Youk, Pedroia etc and it seems like all their stats end up being similar to what they are in the MLB. Not to say that its an instant thing, but from the players I've looked at there is a clear trend. If a player is good enough, they'll adjust to the competition and eventually play the same way they did in AAA and other minor leagues.
Nava has developed enough skill wise, and physically. He was killing the ball in AA, but the only real factor anyone could have against him right now is that he didn't have a ton of time in AAA. He's a switch hitter, he's surrounded by a tough lineup and he is getting his chance to prove himself. If we set an over-under on .780 OPS, I'd pick the over.