Tony DeMarco

Their injury-riddled ace has missed the last 2½ months due to an elbow injury, and may not be back this season, and their star closer just went on the disabled list with a strained right groin.

Their offense is 12^th in the American League in runs scored, and their shortstop — considered as a possible MVP candidate this spring — continues to be dragged down by back troubles, and could return to the disabled list for the second time this month.

So just how have the Oakland A’s gone 27-11 and built a five-game lead in the American League West since the All-Star break? The easy answers are pitching and defense — and because recent history says this is just what the A’s do as summer turns to fall.

Strong pitching comes with the territory in Oakland — that is to say the spacious Network Associates Coliseum, with its heavy night air and expansive foul territory make it one of the game’s best pitchers’ parks.

Only the Detroit Tigers have a lower staff ERA than the A’s, and only the Tigers and Minnesota Twins have allowed fewer runs. Defensively, the A’s are in a three-way tie for second in the AL in fielding percentage behind the Boston Red Sox.

The A’s already had the majors’ best second-half record from 2000-05 — 284-160 for a .640 winning percentage. Their current 27-11 run is even better — a .711 percentage. But it’s not the A’s best stretch this decade. How about 24-4 in August of 2002, which was followed by an 18-8 September for a 103-59 finish and an AL West title?

And in 2001, the A’s were 22-7 in August, 17-4 in September and 6-0 in October — good for 102 wins and a wild card spot. It’s no coincidence that the A’s didn’t reach the postseason each of the last two years, when they were barely above .500 for the final two months.

Digging a bit deeper into this latest streak, three more factors emerge — all the result of general manager Billy Beane’s roster construction.

Last winter, Beane changed stripes and went win-now on us. He gave injury-prone Frank Thomas an incentives-laden deal and an opportunity to continue his climb up the all-time power charts. He handed Esteban Loaiza $21 million over three years — a move questioned by some around the game given Loaiza’s history of inconsistency. And he even dealt a top prospect — NL Rookie of the Year candidate Andre Ethier — to the Los Angeles Dodgers for the ticking time bomb known as Milton Bradley.

In mid-May, Beane appeared to be headed for an o-fer. Thomas sat out four consecutive games when his batting average dipped to .187. Loaiza landed on the disabled list with a strained trapezius muscle after a miserable 0-3, 8.35 April, and Bradley was in the midst of a month-long stay on the disabled list due to a sprained right knee — the first of two DL stays this season.

But Thomas has rebounded, staying healthy enough to accumulate 27 home runs and 75 RBI in 347 at-bats, as well as tie Willie Stargell and Stan Musial for 23^rd on the all-time home run list at 475, and tie Stargell for 37^th in RBIs at 1,540.

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Loaiza (7-7, 5.12) has reeled off three excellent starts in a row, the last a four-hit, 99-pitch complete-game shutout in Toronto on Wednesday. And Bradley has raised his average 57 points and contributed seven homers and 18 RBI since coming off the disabled list on July 14.

Jason Kendall, another veteran whose huge contract ($11.5 million in 2006) was absorbed in a Nov. 2004 deal, also is surging lately. Hitting leadoff during the second-half run, Kendall is batting .336 with 28 runs scored since the break.

But to make up for the losses of Rich Harden, Huston Street and Bobby Crosby, it has taken more — as in the best 25-man roster for the money ($62-plus million) in the game.

The six-headed first base/outfield monster of Nick Swisher, Dan Johnson, Bradley, Mark Kotsay, Jay Payton and Bobby Kielty has spread production and playing time around enough to make up for Bradley’s extended absences and Kotsay’s occasional back flare-ups. Marco Scutaro has been as much a regular up the middle as Crosby and second baseman Mark Ellis, who missed time with a broken thumb.

Kirk Saarloos has stepped from middle relief into the rotation and delivered six quality starts in 12 tries, and six relievers besides Street have a 3.70 ERA or lower. You heard next to nothing about the acquisition of Chad Gaudin from Toronto last winter, but he has a 2.81 ERA in 48 innings in a setup role, joining Street, Kiko Calero and fill-in closer Justin Duchscherer at the back end of games

And as the non-waivers trade deadline passed last month, Barry Zito remained in an A’s uniform. Maybe he wouldn’t have if Harden was healthy and winning games, as there certainly was plenty of interest in Zito in an otherwise pitching-poor market. But the A’s hung onto the last of the Big Three, and will take their chances in the postseason, then let him go to free agency and take the draft picks in return.

Maybe they could have reached the playoffs without Zito (and in essence, Harden), but advancing beyond the first round for the first time in four tries this decade is the idea here, and the AL is stacked with arguably eight of the game’s top 10 teams.

So for now, Zito has stayed put, the A’s have won 10 of 13, 19 of 24 and 27 of 38, and haven’t lost a series in their last eight. It’s that time of the year again, after all.