I never thought I would hear Trout and Nava mentioned in the same sentence. Trout could be the worst defender in MLB history, and still would be an elite player. There's also a huge difference between poor (2011 Carl Crawford), and historically bad defense (2013 Daniel Nava). We should be fortunate that Nava isn't a terrible baserunner.
I'm not worried about the ten game sample size. He'll definitely improve on his start. The question is how much he will improve? I am not optimistic about a repeat of his LD% and BABIP this year. How many players have similar skills to Nava, and consistently have huge years? Nava does not have elite patience at the plate. A BB% of 9.5% is above-average. He isn't going to leg out a lot of infield hits, and he has limited power. The only way he sustains his BABIP is continued luck. Look at the players with high LD% and high BABIP:
http://www.fangraphs.com/leaders.aspx?pos=all&stats=bat&lg=all&qual=y&type=c,4,6,-1,34,35,43,40,41,53&season=2013&month=0&season1=2004&ind=1&team=0&rost=0&age=0&filter=&players=0&sort=8,d
Not many repeat visitors at Nava's level. Not many players that are similar to him either. There's also some bad performances despite the high LD%. The only hope is that he continues to get hit by a lot of pitches. He finished with 15 HBP, which was 4th in all of MLB.
I don't have any faith in his defense improving. Nava has rated as having awful range every single year in the majors, and in two of those he's historically bad. Nava has one and a half above average skills. He can draw some walks, and can hit right handed pitching. He's a pinch hitter at best. Carp already does that, and Gomes is a legitimate masher against lefties. Bradley can play all three outfield positions. Nava is the one who should go, but he'll stay up because of last year.
No, but he's better than Nava. I think Sizemore, Bradley, and Victorino is the eventual outfield. This team might be short of power come July, so I'd kick the tires on Carlos Gonzalez and Stanton. Maybe the Marlins don't want to pay Stanton the rest of his $6 million.