Well it has taken Nava a long time to get anywhere because he has used all of this time to become a really smart baseball player. He must have been like a sponge this whole time. That is why he is here. He has studied hard, learned well and puts all of it to good use.
So what he has is not likely to go away. He has a nice compact swing and he takes good swings at good pitches to hit. Goes the opposite way when he should. You can tell that he gets overpowered by guys that the really talented hitters can still hit so that is his ceiling if you will.
I don't believe he will hit this good for a major league career or even a full season. For one thing not only can he be overpowered by somebody that just has good stuff (does not have to be great stuff), he likely has a spot in the strike zone that he just cannot get to, where he can be overpowered pretty easily. That is a spot where with two strikes, he has to protect the plate and just cannot get to a particular pitch.
All of that said, based on what I have seen so far, I have no doubt that he can start for somebody in MLB. Just look at the teams that are not that good and the guys playing for them. Then look at the teams that would love to have Nava as their backup outfielder. So to me he is either a starting OF for some teams or a backup OF for others. However unless there is some flaw in his game, yet to be exposed that is so weak that once discovered really makes it easy to get him out, this guy is a ML OF to me.
You don't see many players like him.