These numbers don't add up

This spring, when Epstein said, "I think the game is 50 percent run production and 50 percent run prevention," it was understood what he meant. Epstein values fielding and slugging as equal means of winning games, and he assembled the Red Sox this winter as a team that could rely on its defense.

So it comes as a surprise that 63 games into their season, the Red Sox have been one of the worst defensive teams in baseball. The Sox have made 38 errors, 12th most in baseball, and their .983 fielding percentage ranks 19th. And deeper metrics suggest more of a struggle.

"I would say that we've been spotty at times," manager Terry Francona said. "I do know we care a lot about defense."

The Sox' defensive issues have not prevented them from ascending to the top of the American League. The Sox just completed a 9-3 streak against four teams that were in first place when the series began, and, at 38-25, they trail only the Los Angeles Dodgers for baseball's best record. They may not be winning how Epstein envisioned, but the Sox are winning.

"We're in first place by [two] games," second baseman Dustin Pedroia said. "I don't think anyone gives a [care] what the numbers are."

For those who do, they are mostly bleak. The Sox are fourth worst in baseball and second worst in the AL in runs prevented, according to the Ultimate Zone Ratings on fangraphs.com, a website devoted to baseball statistics. The UZR considers range, errors, and arm strength, denoting a value, in runs, to each. By the formula, the Sox have prevented 17 runs below average. For comparison, the league-leading Tigers are plus-33.2.

Statistics show what any close follower would expect - the left side of the Sox' infield has been a liability and the main reason for the poor rankings. Shortstops have combined for a -7.0 UZR, and Julio Lugo by himself is -5.3; only Yuniesky Betancourt of the Mariners is worse among AL shortstops. Nick Green has made nine errors, tied for the most among major league shortstops.

At third base, Mike Lowell has prevented 6.2 runs below average; only Michael Young of the Rangers is worse in the AL. Lowell has been uniformly average in all fielding aspects save one: range. He remains a slick fielder, but offseason hip surgery shaved a step or two off the ground he covers.

"I think there's usually more into that than just what meets the eye or what the numbers say," Francona said. "Our shortstops have made a lot of errors. Saying that, because Greenie has played the majority of the games, I think it's a different bar. Mikey Lowell coming into the year was slower than he is now. We were fighting some things early on. I think we're better."

The Sox have reason for optimism for both positions. Jed Lowrie could return at short by the weekend. In 49 games last season, Lowrie committed zero errors in 155 chances and posted a plus-5.9 UZR. Lowell has gained speed little by little as the season has worn on and his range will likely improve as his hip improves.

"Our defense should improve as we get healthier," Epstein said.

The improvement helps explain, in part, the resurgence of the starting pitchers. On May 15, starters had a collective 5.96 ERA. Since then, they've posted a 3.59 ERA while going 17-10. The starters are largely responsible - defense didn't help Jon Lester strike out 11 batters in each of his past two starts - but one ground ball in the hole that may be either a two-out RBI single or an out makes a difference.

"Most definitely," Francona said. "It's hard to have crisp, clean games where our starter goes deep if you're not catching the ball or throwing it where you're supposed to. When you don't catch it and don't make plays, it's hard to have a real good staff. There were a lot of games where it seemed like guys were one pitch away, or we couldn't make one pitch where we ended up giving an extra out. That happened a lot.

"Defense . . . sometimes it's a hard thing to measure. But I know if you give extra outs or extra opportunities, you're asking for trouble."

Adam Kilgore can be reached at akilgore@globe.com