This has been an interesting topic to me all year. Judging things like route-running on TV is really hard, so for most of the season I didn't really think a whole lot about McLouth's fielding. I saw that he'd made no errors all year and just kind of took a pass on worrying about it. When I came home in June, the one thing I noticed in the two innings of the Yankees game that got rained out was that it seemed to me, from my vantage point on the right field wall, that he was getting awful jumps on balls hit over his head. I noticed the same thing from left field the next day at the Rays game and then noticed it again on TV about a week later, at which point I mentioned it and was brandished for drawing a judgment from only seeing one or two games in person, so I dropped it.
Still, the thought has stuck in my head for a bit and I noticed that today Dejan mentions it in his morning link dump. Specifically, he links to this post at BBTF, which discusses defensive runs saved and says this about McLouth:
One of the surprise hitters of the year, Nate McLouth (-15), has been absolutely terribly (sic) with the glove. McLouth may be better suited to a corner position.Dejan responds with this:
The same McLouth who is joined only by Torii Hunter among center fielders with zero errors in Major League Baseball. I understand there are all kinds of range factors and things like that to dissect, but let this absurd assessment specifically aimed at McLouth's glove underscore the great distance still left to be covered with fielding statistics. McLouth has been very, very good in all facets ...He goes on to list the things he's observed McLouth to be very good at. It's a bit at odds with what I've thought of McLouth's defense, but I've seen two games plus part of a rainout this year, while Dejan's seen most of them from the press box.
Before I launch into semi-nerdy fielding talk, if you're unfamiliar with things like zone rating, check out the presentation that David Gassko and MGL of The Book put together on defense. It's in straightforward HTML and it's a very good primer. Judging any fielder by errors is a very poor way of measuring fielding, but judging an outfielder by errors made is about the worst way possible. Look at the fielding percentages for the qualifying centerfielders in the NL compared to fielding percentages for qualifying shortstops. It's just harder to make an error in the outfield, unless you're Rick Ankiel. But then, look at RZR for centerfielders. RZR is the revised zone rating compiled by John Dewan, based on balls fielded in an outfielder's zone. McLouth is last by a mile among NL Centerfielders. There's a huge gap between Lastings Milledge's .880 and McLouth's .859. According to Dewan's measurement's, Nate's only made 207 of 243 plays in his zone. He is, however, fourth in the league with 56 plays made out of his zone. Since zone rating doesn't account for fielder positioning, it's hard to know exactly why the two stats don't agree. If you want to look at other fielding metrics, he's 20 runs below average in BP's FRAA system and Baseball-Reference has him at slightly below average in range factor per nine innings.
None of these stats are perfect. RZR only takes into account play in zone, there are some serious criticisms of FRAA that I can never remember without a primer, etc. A better way to judge fielding is using something like UZR (ultimate zone rating), which takes into account both in zone and out of zone plays. When the guys that compile it (that would be MGL from The Book) last updated in early June (if you can find a more recent update, let me know), McLouth wasn't listed as being among the best or worst centerfielders in either league, though some AL butchers, Andruw Jones, and Jim Edmonds clogged up the bottom of the list. That means that McLouth could well still be below average and at the bottom of the NL.
It seems to me that McLouth is probably slightly below average in center field. Even if most fielding metrics aren't perfect, they all seem to have him below average and even accounting for his high number of out of zone plays, it's hard to reconcile those other numbers completely when I've seen him break very poorly on balls hit over his head with my own eyes. There shouldn't be a debate about who plays center when Andrew McCutchen is ready.