If you expand that FG leaderboard to a three-year slice to get a better sample, JDM is no longer one of the very worst throwing OFs, but he does acquire the 7th worst range among the 51 qualifiers. And he's near the bottom in error runs as well. Speaking of which, I don't think I want a guy whose errors are mostly of the "mental/lack of concentration type" patrolling the Monster.I think we ought to look a bit harder too at what is causing such a drop in fielding value for JDM. I'm sure his range has dropped - but that's not the biggest issue. He can't throw anymore.
http://www.fangraphs.com/leaders.aspx?pos=of&stats=fld&lg=all&qual=y&type=1&season=2017&month=0&season1=2017&ind=0&team=0&rost=0&age=0&filter=&players=0&sort=19,a
It has dropped from nearly 9 runs above average to 8 runs below average in three years. That means teams are running freely from 1st to 3rd or 2nd to home. That isn't going to get better in RF, but it's going to help a lot playing LF in Fenway. Those also skew the numbers.
He's also had the dropsies more than usual the last two years with 10 errors. There was an article which I can't find now that had a video montage of all of them and most of them were of the mental/lack of concentration type.
There's no sugar-coating it. He'd be terrible out there. Of course, we've survived bad defensive left fielders before -- in fact we've won three championships with them. But if they put him there, let's not pretend it isn't gonna be ugly.