MiLB has a nice article on (hopefully-) 2013 LF J.D. Martinez, where he addresses anyone critical of his fielding ability:
"Well when you get drafted in the later rounds like I did, it's always because the scouts see something they don't quite like. I'm not a five-tool guy, few are, but in no way am I going to drop balls out there. The Astros know I can play defense, and I've been working with Milt Thompson, our roving outfield instructor on my routes to fly balls. I'm confident that I can get to balls now, and the Astros have noticed I've improved out there."
Honestly, who wouldn't rather see J.D. Martinez than Carlos Lee in LF?