Yonder Alonso has been one of the few bright spots this season for the San Diego Padres. Acquired in the Mat Latos trade, he currently sits fourth on the club in OPS+ among those players with 140 PA or more. Yet if you look at his Fangraphs page, his 2012 season so far grades out at -0.3 WAR. Why? Two reasons: he's a bad baserunner, and he's not so good defensively.
So just how bad is Alonso's defense? Well, his UZR as of this morning is -2.4, and his UZR/150 is -6.4. Numbers in a vacuum don't mean anything, so I looked at first baseman with at least 400 innings played this year (Alonso has played 483 innings). On that list, the best defensive 1B so far in 2012 is James Loney. Alonso is not the worst defensive first basemen - that honor belongs to Kansas City's Eric Hosmer - but he's in the bottom half of the league.
Not surprisingly Prince Fielder shows up at the bottom as well. Suprisingly so does Casey Kotchman.
Can we see what specifically Alonso struggles with? For that we have to go to Dewan Plus/Minus. According to those numbers, Alonso is -2 on balls to his right, -4 to his left, and +1 on balls hit right at him. Overall it's a -5. Balls in the air haven't given him any trouble. Looking at the Runs Saved statistics, his defense has allowed two more runs to score than the average first baseman, and places him 28th in the league.
Being left-handed, it makes a little bit of sense that he'd be weaker trying to field balls hit down the line than otherwise, but it can't be used as an excuse. Adrian Gonzalez is left-handed as well and by the same metrics is actually BETTER fielding balls hit to his left than to his right. Of course he's also the second-best defensive 1B according to that list I linked to above, so feel free to scream into a pillow or punch a Red Sox fan in frustration.
The good news is Alonso's only 25. Defensive is something that can be improved as players get older; being a lousy defender now is not a death sentence. As the Padres coaching staff works with him he will improve, but these things take time. He can improve his value dramatically if he'd quit getting doubled off first base on fly balls to the outfield, but that's a post for another day.