I would assume that he is referring to Fangraphs, which had him as the 9th best SS in all of baseball based on their defense component of fWAR, right between Tulo and Iglesias.If he's really an "above average defender," then his defense is likely to measure higher if anything, since his advanced defensive numbers were essentially average by both UZR (1.0) and DRS (0). I'm not sure where you got the idea that he had the numbers of "one of the best defensive shortstops in baseball."
I think an average defensive shortstop is exactly what he is at this stage in his career, so I think the likeliest result is little to no change in his dWAR, though of course we all know how volatile those numbers are from year to year, so who knows.
link: http://www.fangraphs.com/leaders.aspx?pos=ss&stats=fld&lg=all&qual=y&type=1&season=2015&month=0&season1=2015&ind=0&team=0&rost=0&age=0&filter=&players=0