Sox manager Terry Francona argues a call with first-base umpire Mike Mulchinski.

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AP / Pat Sullivan

HOUSTON — Blame it on the law of averages. And while you’re at it, label last night’s 11-10 loss to the Houston Astros for what it was — one of the most frustrating losses of the season for the Red Sox.

Within the span of a few innings, the Red Sox’ most reliable starter over the last two months, Jon Lester, blew a 4-0 lead, and the team’s bullpen, having allowed only one run in the previous 24 innings, gave up a three-run edge.

The failings of the pitching staff obscured a 13-hit attack by the offense, led by Mike Lowell who had four hits and three RBI and another huge night for Dustin Pedroia (four hits).

“We did some good things offensively,” said Terry Francona. “We just couldn’t keep them off the scoreboard.”

The Sox led, 9-8, in the bottom of the eighth when Manny Delcarmen, who had gone more than a month since allowing his last earned run, yielded a game-tying homer to Ty Wigginton.

It got worse after that. Pinch-hitter Darin Erstad singled, was sacrificed to second and was joined on base an out later when Hunter Pence walked. Lance Berkman then laced an opposite-field double to left, scoring both base runners.

Lowell hammered a solo homer in the top of the ninth off closer Jose Valverde, but that proved to be the last — and futile — comeback effort.

“Kind of tough,” summarized Delcarmen, who had retired the last 14 hitters he faced and been unscored upon over his last 11 appearances, “but that’s baseball. It won’t be the last time.”

Delcarmen hung a changeup to Berkman, part of a night-long problem for the Boston pitching staff.

“We made some mistakes over the middle of the plate,” Francona said, “and paid for [about] every one of them.”

The loss snapped a modest three-game winning streak for the Sox overall, and a three-game road winning streak, too.

The last time the Sox scored 10 or more runs in a loss was Aug. 18, 2006, in a 14-11 defeat to the Yankees.

While Delacarmen absorbed the loss, the bullpen problems first sprouted in the seventh.

David Aardsma enjoyed a clean inning in the sixth in relief of Lester, striking out two to give him 10 strikeouts in his last 12 recorded outs.

But in the seventh, Aardsma invited trouble by walking leadoff man Michael Bourn and followed that by allowing a single to Pence, giving the Astros runners at the corners and no out.

Craig Hansen took over and gave up a run-scoring single through the box to Berkman, scoring Bourn. A passed ball by Jason Varitek allowed Pence to score before Hansen steadied himself and retired Carlos Lee (flyout), Miguel Tejada (groundout) and in an epic 11-pitch at-bat, Mark Loretta (flyout).

That came after the Sox had erupted for five runs in the sixth, taking advantage of a tiring Brandon Backe, a miscue in right by Pence and poor relief work from Geoff Geary.

With Julio Lugo on first and one out, Jacoby Ellsbury pulled a pitch to right on which Pence mistakenly charged. But the ball soared over his outstretched glove and rolled to the wall for a triple, scoring Lugo.

Backe left — but not before glaring at Pence in right on his way to the dugout — in favor or Geary, who proceeded to let four of the next five Sox hitters reach.

Pedroia’s fourth hit scored Ellsbury to pull the Sox even. J.D. Drew then singled to right as Pedroia took third and when Manny Ramirez walked, the Sox had the bases loaded.

Lowell then drove a fly ball to right, deep enough to score Pedroia and give the Red Sox their first lead of the night.

Youkilis, who has been nearly as hot as Pedroia, then lashed a double to the gap in left-center, scoring both base runners though Youkilis made the third out of the inning when he tried to advance to third.

Lester, who had not allowed more than three runs in any of his previous 11 starts, allowed five in the third inning alone after the Sox had staked him to a 4-0 lead in the top of the fourth.

Lee drilled a two-run single to get the Astros on the board and Tejada followed with a shot up the middle that caught Lester’s left heel before bounding away for an infield hit.

Francona and trainer Paul Lessard came out to check on Lester, who remained in the game. But on the very next pitch, the lefty served up a three-run homer to Loretta, a former teammate.

“(The ball off the heel) kind of knocks you for a loop,” said Lester, declining to use it as an excuse for what followed. “But you’ve got to tip your hat to Loretta — he was looking middle-in. I threw the ball there and he put a good swing on it.”

smcadam@projo.com