The Indians can get away with experimenting with longtime second baseman Jason Kipnis in center field for the same reason they got away with putting third baseman-turned-outfielder Lonnie Chisenhall there earlier this year and first baseman/designated hitter Carlos Santana in left field in the World Series last year: Because their dominant pitching staff allows Terry Francona the flexibility to do so.
That's another way of saying "if there aren't likely to be a large amount of competitive plays hit to a spot, you don't have to worry about defense so much," which mirrors the effect the infield shift has had on baseball's ongoing home run surge. Among many other reasons for more dingers, fewer difficult balls in play due to more strikeouts and better positioning has allowed power bats like Carlos Correa and Corey Seager to remain at shortstop, where a generation ago they'd likely be third basemen by now.
It's the same idea for Cleveland. Kipnis, who last played center field in college and wasn't challenged with a play in his first game there on Sunday, may or may not turn out to be a competent outfielder. (Chisenhall did; other infield conversions, like Hanley Ramirez, worked out less favorably). But with Bradley Zimmer injured and untested rookie Greg Allen (who had only a .701 OPS in Double-A this year) the primary alternative, they can at least afford to take the risk in order to get Kipnis' bat in the lineup while keeping the trio of Jose Ramirez, Giovanny Urshela and Yandy Diaz at second and third.

To start with, this is about strikeouts, and Indians pitchers are piling them up better than any team in history. The obvious caveat there is that every year is the biggest strikeout year these days; the top five strikeout teams this year are also the top five strikeout teams ever. But that's sort of the point; no team has allowed a lower percentage of opponents to put the ball in play. Whomever is standing behind Corey Kluber, Trevor Bauer and friends is less likely to see a batted ball than most players who have ever played.
That doesn't mean the Indians can completely punt on defense, of course, and they haven't. The Tribe's transformation from one of baseball's weakest defensive units in 2013-14 to one of the stronger ones since corresponds well with Francisco Lindor's arrival, and it's been an underrated part of the Indians' ascension to regular title contenders.
But we're not talking about defense all over the field, when it comes to Kipnis. We're talking about center field. Thanks in large part to the pitching staff's ability to prevent any type of batted ball, the Indians' center fielders simply haven't had a ton of balls hit their way -- just 357, the sixth lowest in baseball. That's well below the average of 395, and it's a whopping 139 below what Byron Buxton and his Twins teammates have seen in center. We're talking 2.4 plays per game. It's not a lot.
Still, we can do better. That number is showing any ball that's theoretically catchable, which is to say a Catch Probability above zero percent, but there's an enormous difference between a ball that's virtually always caught (99 percent Catch Probability) and one that's virtually never caught (one percent). We're not worried about the extremely difficult balls (25 percent Catch Probability or under) that most outfielders can't get. We're not worried about the very easy balls (75 percent Catch Probability or over) that most outfielders, including Kipnis, get pretty easily, like this simple 91 perent grab that Zimmer made against the Royals:

Instead, we're most interested in that middle band, the balls with a Catch Probability between 25 percent and 75 percent, which is where outfielders really set themselves apart (or don't). Batted balls to outfielders within that band are turned into outs just under two-thirds of the time. These are the ones that are important to be converted into outs, and as far as center field goes, there just aren't that many in Cleveland.

That's 28, compared to 60 for Minnesota (which has Buxton) and 52 for Atlanta (which has Ender Inciarte). After the third-place 50 for last-place San Francisco, struggling with Denard Span, you see Kansas City (Lorenzo Cain) and Cincinnati (Billy Hamilton). If you're allowing a lot of competitive plays to center, you basically have to have a plus center fielder. If you're not, it doesn't matter quite so much. (As you can see, the Dodgers are near the bottom, and after Joc Pederson underperformed, they've patched with career infielder Chris Taylor and rookie Alex Verdugo.) We're talking one of these plays every few days, at least for Cleveland's center fielders.
"[Kipnis is] going to play a lot of games [in center]," said Francona last week, and you understand why. With Zimmer out, and the infield overloaded, you need a way to get Kipnis back in the lineup, and with this pitching staff, it's lower-risk than it might be for a team like Minnesota. Sure, Cleveland saw some adventures from Tyler Naquin in center last October; there's not zero risk. The Indians didn't get here by being completely traditional, though. So far as risks go, this is a smart one to take.