Jim Riggleman & Company have a lot to think about over the next few months, not the least of which is how to improve the Nats sloppy defense — first in the majors in errors — and what to do with Adam Dunn. The two are closely related, particularly given the questions being raised about Dunn’s prowess at first base. Ben Goessling (in “The Goessling Game“), focused on Dunn’s defense in his latest blog, noting that the Nationals “have made no secret of the fact that Dunn’s defense is the main thing they’re still evaluating when deciding whether to give the 30-year-old a contract extension.” But Goesseling praises Dunn for making the successful switch to first base, implying that his defensive play has been one of the surprises of the year: “To Dunn’s credit, he’s improved markedly at first base this season, becoming a slightly below-average fielder instead of an anemic one,” Goessling writes.

Goessling isn’t the only one praising Dunn. Over at SI, Joe Sheehan provides a list of players who are among the worst defensive players in the game (Yuniesky Betancourt, Brad Hawpe, Hanley Ramirez, Andre Eithier and Ryan Braun), while noting that Dunn “has adapted well” to being moved from the outfield to first base “showing good hands if limited range.” Dunn’s successful shift is unusual, Sheehan writes: “It’s rare that a player can move to first base and increase his value, but Dunn has done so.” So if Goessling and Sheehan are impressed with Dunn — and if the big guy from Texas is ripping the cover off the ball (and he is) — what’s the problem? Well, the problem seems to be Friday night in Philadelphia, when Dunn’s lack of range hurt him in getting to a ball stroked down the right field line and where the big man’s size limited his stretch to snag a ball thrown by Ian Desmond. The Goessling-Sheehan notes on Dunn are now the subject of some attention on the web (as MLB Trade Rumors runs through the Nats’ first base options) and increased questions over the utility of “defensive stats.”

So  . . . how concerned should the Nats be with Adam Dunn’s defense? Dunn has made seven errors in 116 games playing first for the Nats, a statistic that actually reflects the common judgment that Dunn is a below average first baseman. Derrick Lee has made six errors (in 106 games), while Joey Votto has just four in 112 games. Lee and Votto ought to be considered the class of the league (Lee is tall, agile with lots of range; Votto is young and tough with great leaping ability), but the best in baseball (at least according to this single, and admittedly limited, “errors and chances” measure) might well be Friars’ first sacker Adrian Gonzalez who seems to have everything — agility, range, height and experience. Gonzalez has just five errors in 118 games, and he’s there, day-in and day-out. Which is not to mention Albert Pujols (with just three errors), who has everything that Gonzalez has (but perhaps not as much agility), or James Loney (a Gonzalez without the range, it seems to me) — with an astonishingly nearly perfect three errors in 122 games.

So the critics are right: Adam Dunn is an average to below-average first baseman: he ranks below Derrick Lee, Joey Votto, Adrian Gonzalez and James Loney, is on a par with Atlanta’s gimpy and aging Troy Glaus, but is a ton better than either Arizona’s Adam LaRoche (10 errors in 112 games) or Philadelphia’s Ryan Howard — whose defensive gaffes are legendary (11 errors in 101 games). The problem with all of this is that defensive stats don’t really tell us a lot, which is why Sabermetricians have struggled to come up with better ways of measuring defensive prowess. But, as Tim Marchman noted recently, the statistical models are controversial, contradictory, and often fly in the face of common sense. In truth, it’s nearly impossible to compare Dunn’s defense to Pujols’ or Loney’s or anyone elses, because only Dunn offers a glove to Ian Desmond — whose lack of experience regularly (viz. Friday night in Philly), makes Dunn look like Adam LaRoche (whose own shortstop, Stephen Drew, throws baseballs as naturally as the rest of us eat chicken).

That’s not to say that fans of Adam Dunn should ignore his defensive woes, or refuse to admit them. It’s only to note that, when it comes to Dunn (and anyone else playing in the field), defensive stats will either only confirm what we already know (do we really need a raft of stats to tell us that Adam Dunn lacks range, experience and agility), or will stand as a confusing single data point in an overall picture (Colin Wyers over at Baseball Prospectus points this out, and pretty convincingly). Which leads us back to where we started — with this single question: do Adam Dunn’s offensive stats (his home run, OBP and RBI totals) compensate enough for his defensive woes to make Riggleman and Rizzo think about signing him for a few more years? The answer, as always, depends on the option — of whether Dunn’s prospective replacement will improve the Nats defense so markedly that they can live without the 40 or so home runs that the Texan will hit.

Ben Goessling says that the Nats are thinking along these lines, by considering Carlos Pena as a useful replacement for Dunn at first. Pena has the same kind of pop (39 home runs last year, 46 in 2007), and while he’s more experienced and more agile at first, it’s hard to argue that he’s actually better defensively (10 errors in 133 games in 2009). Perhaps more importantly, it’s hard to argue that Pena’s more resilent. The savvy Tampa first sacker sat out most of 2005 and nearly all of 2006 with injuries, is known to be plagued by an inexplicable June injury bug — and has been hit with broken fingers, pulled hamstrings and swollen foots. And Adam Dunn? The last time that Adam Dunn had any kind of injury at all (knock wood, right now, and knock it hard) was 2003. Over the course of the last seven seasons, Dunn has played in (count ‘em) 161, 160, 160, 152, 158, 158 and 159 games. That’s the real stat, the one that matters. In comparison, in that same stretch, Carlos Pena played in 131, 142, 79, 18 (18!), 148, 139 and 135 games. Nearly a full season less. Maybe Pena is better at first. Maybe. But it’s hard to make an error when you’re sitting on the bench.

So if all of this is true, what’s all the hubbub about Adam Dunn? And why, now, are we suddenly hearing about Carlos Pena? The answer might be that the Nats really do want to get better defensively — and they think the way to do that is to replace their 40 homers a year guy with a player (like Pena) with a puzzling history of nagging injuries. Or maybe, just maybe, all of the complaints about Dunn’s fielding have nothing to do with his defense at all. After all, there seems to be a trend here, and it has more to do with the bottom line than booted balls — and should be perceptible to anyone who pays close attention: when the Washington Nationals’ front office starts talking about replacing Dunn’s below-average glove at first base, what they’re really talking about is replacing his big salary in the accountant’s book.