It's possible that more has been made of defensive positioning over the last five years than the prior hundred before it. Infield shifts are something we actually track now, and if the early season is any indication, the usage of those infield shifts are on the climb for a fifth consecutive season. Players and managers discuss them openly, television broadcasts take note -- some going so far as to display the position of every fielder on the screen -- and we see the benefits (and occasional drawbacks) of the shift on a daily basis.
But nearly all the attention we've paid to defensive positioning has gone to the infield. There're more holes in the infield, less margin for error when the shift doesn't work, and baseball is slow enough to adapt to any sort of change that it should come as no surprise we had to take this one step at a time. But now, slowly but surely, teams have begun shiftingmore in the outfield, and before long, the outfield shift, just like the infield shift, will become accepted as standard practice, rather than something that demands attention when it happens.
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But shifts aren't confined to lateral movement, the way we most often see. Players are free to move in and out, too, and thanks to new Statcast™ data, this is the kind of thing we're starting to see measured and quantified.
Enter Dexter Fowler. Fowler was the center fielder behind Jake Arrieta for Arrieta's no-hitter on Thursday night. Fowler's been arguably the best player in baseball this year, owing to positive marks in the batter's box, on the bases and in the field. Fowler's long been an above-average hitter, and he's long been a plus baserunner, but the defensive marks haven't been so kind. You could make the case the defensive marks are the thing that's prevented Fowler's reputation from exceeding "nice little player" to "borderline star." An above-average hitter who runs the bases well with a reputation as a plus center fielder is a borderline star. But Fowler hasn't had the reputation as a plus center fielder, because the tools we use to evaluate defense in this day and age have considered Fowler one of the worst defensive outfielders in baseball since he entered the league.
Since 2009, Fowler's first full season, 28 players have registered at least 3,000 innings in center field -- the kind of sample size we prefer to have before working with defensive metrics. To contextualize Fowler's place among his defensive peers, I took that pool of 28, weighted Defensive Runs Saved and Ultimate Zone Rating equally, and prorated the figures to roughly a year's worth of playing time. The three lowest-rated regular center fielders over the last seven years, by the numbers, are as follows:
1. Matt Kemp, -11 runs saved per 1,000 innings
2. Fowler, -9
3. Angel Pagan, -4
Fowler stands 6-foot-5, 195 pounds, has good speed, and overall looks like an elite athlete, so his consistently league-worst defensive metrics have always been puzzling, the kind of guy the eye-test crowd uses as an example against the metrics by pointing at him and going, "Look at him!" Which, I can't blame them. Fowler looks like he should be fine defensively. It's always puzzled me, too.
Which is why I was immediately captivated by this tweet from Mike Petriello last month:

It shed some light on one way in which Fowler is a defensive outlier in center field, one potential piece of the puzzle toward explaining the differentiating opinion on his defense. The pairing with Adam Jones was particularly intriguing, considering Jones is another guy whose athleticism belies the data's opinion of his range.
It should be noted, as Petriello later pointed out, that Wrigley Field has one of baseball's smallest outfields, with a particularly shallow center field, and once we adjust for that, Fowler isn't quite as extreme an outlier. But it's not like Fowler doesn't have a history of erring on the side of shallow. When he came up with the Rockies, he was instructed to play shallow, a philosophy that stuck with him even after leaving Colorado.
But the Cubs have different ideas. The Cubs have instructed Fowler to play deeper in center field this year, with manager Joe Maddon citing "defensive metrics" as the reason why. Research done by John Dewan, the owner and chairman of Baseball Info Solutions, showed that the runs saved by center fielders with a tendency to play shallow on balls coming in was far outweighed by the runs lost on balls over their head -- Fowler being one of the most extreme examples of this. It's not hard to hypothesize why. Miss a ball in front of you, you likely give up one base. Miss a ball over your head, you're likely giving up two or more.
So, back to the no-hitter. It was the eighth inning, and Arrieta's no-hitter was obviously still intact, considering he still hasn't given up a hit since. It was the eighth inning, there was one out, Devin Mesoraco was at the plate, and he hit a ball to center field that looked like this:
Gif: Dexter Fowler catch in center field
Can of corn! Certainly nothing that stuck out at the time. Pretty routine catch. Routine for someone positioned where Fowler was positioned. Maybe not so routine for someone who played one of the most extremely shallow center fields of anyone in baseball, like Fowler has in the past:
Gif: Billy Hamilton Triples
Granted, no two batted balls are hit the same, and we're not saying that both batted balls were identical. But it's clear to see the extreme change in Fowler's positioning from this year to last, and the ball he caught last night was hit to dead-center field. Look at Fowler's positioning in Cincinnati last night, compared to the example from last year:


Just as no two batted balls are alike, no two batters are alike either, and of course Fowler is going to play Mesoraco a bit differently than he plays Billy Hamilton. But I put a little yellow dot where I suspect last year's Fowler, prior to the coaching staff's instruction for him to play deeper, might have played Mesoraco. Maybe 10-15 feet more shallow. Now watch the GIF from Thursday night's no-hitter again, and pretend Fowler's starting position is 10-15 feet closer to home plate. Suddenly, he's going back. Suddenly, he's going back and trying to catch a ball hit almost directly over his head. Trying to catch a ball hit directly over your head on the fly is one of the toughest catches in baseball to make.
Do we know that my yellow dot is a perfect indicator of where Fowler would've played Mesoraco last year, when he still played a shallow center field? Of course not. And even if he had started at the yellow dot, is it possible that he could've made the play anyway? Sure. But it would've been a lot more difficult. Thursday night, it wasn't difficult. Thursday night, it was routine, because Fowler was playing deep and didn't have to worry about the ball going over his head.
It's not a guarantee that playing deeper will make Fowler a better center fielder. But Maddon and the Cubs seem to think it will, and at the very least, it might've saved Arrieta's no-hitter.
A version of this article first appeared at FanGraphs.com. This story was not subject to the approval of Major League Baseball or its clubs.