Almost without warning, the night went horribly wrong for the Red Sox, who had a 6-2 lead after six and a 7-4 cushion entering Papelbon's frustrating frame -- the ninth.
The night started in feel-good fashion for the Red Sox, with freshly-minted Hall of Famer Jim Rice having his No. 14 retired to the right-field façade at Fenway.
But the hits kept on coming after that, as the Athletics drilled 21 of them against the Sox. The biggest dagger was the 20th, delivered by Rajai Davis, who struck a two-out RBI single to right in the 11th to score Mark Ellis from second to snap the tie and give the A's the lead for good.
"It was a breaking ball 3-2, down-and-away, and he just happened to hit it to the right side, and it got through," said Manny Delcarmen, the reliever who gave up the hit.
Fatigue might have been a factor, as Delcarmen threw 34 pitches over his 1 2/3 innings.
"I thought Manny was a little bit fatigued at the end, and that's not his fault," said Red Sox manager Terry Francona. "I thought he gave a great effort -- he just gave up a hit."
So did Takashi Saito, who came on and allowed an RBI single to Adam Kennedy to pin the Sox in a two-run hole.
Still, there was hope in the bottom of the 11th. Mike Lowell and George Kottaras led off with singles against A's closer Andrew Bailey. Nick Green pushed the tying runs to second and third with a bunt. Jacoby Ellsbury made it a one-run lead with a grounder to first, and the reigning American League Most Valuable Player stepped to the plate.
But Dustin Pedroia didn't quite get all of it, flying to deep left to end the game.
The beginning of the end, however, was definitely the top of the ninth. Jack Cust led off with a walk, which was made only more painful because Papelbon had started him 1-2. Cust moved to second on defensive indifference and to third on a grounder to second. Just one out from victory, Papelbon surrendered an RBI double to Tommy Everidge, who was playing in his first Major League game.
"The leadoff walk was what really set the tone for that inning," said Papelbon. "I had good stuff, and they were able to put together some at-bats. I felt like the only at-bat that I didn't really make a good pitch was to [Everidge]. I have two outs there. I have to finish that game."
Still, though, the Red Sox had the comfort of a two-run lead. But Ellis followed with an infield RBI single that Green exacerbated with a wild throw to first that sailed out of play for a two-base error. Davis then tied the game on an infield single to Green, who again made a throwing error, though that one didn't lead to a run.
"It's a play nobody makes," Green said of the ball hit by Ellis. "It was just not the right decision. I should have just held it."
It was a night full of regrets for the Sox, who lost an opportunity to chip the Yankees' lead to 1 1/2 games in the AL East.
"It's always tough," said Green. "A three-run lead with Papelbon, we think we're going to win every time. That's the way it goes sometimes, and it stinks, but we've got to bounce back tomorrow and try to get back on track."
The Red Sox didn't seem immediately deflated by the rare blown save by Papelbon. In fact, Jason Bay made a tremendous diving stop that ended the 10th and robbed Kurt Suzuki of a go-ahead RBI single.
And in the bottom of the 10th, J.D. Drew nearly sent everyone home happy, pummeling a drive to deep right. But instead of sailing into the bullpen for a walk-off win, Drew's drive settled into the glove of Davis, just in front of the wall.
"I think we all thought he had a chance for a home run," said Francona. "Maybe we're hoping, but it looked like it had enough."
The Boston bullpen, which hadn't allowed an earned run the first 10 games after the All-Star break, had no answers in this one.
The A's struck for a run against Ramon Ramirez in the seventh and another against Hideki Okajima in the eighth.
The Red Sox put up a five-spot in the bottom of the third to stake starter Clay Buchholz to a 5-2 lead.
With the bases loaded and one out, Kevin Youkilis tied the game by lining a two-run double into the corner in left. David Ortiz gave the Red Sox their first lead on a grounder to second that scored Pedroia. With two outs, on what looked like a half-swing, Bay reached on an infield single that brought home Youkilis. Drew capped the inning by ripping an RBI double to right. Suddenly, Boston had a 5-2 lead.
The Red Sox bumped the lead to four runs in the sixth, as Lowell smashed a double into the corner in left that scored Drew all the way from first.
Buchholz wasn't spectacular, but got the job done, giving up nine hits and two runs and throwing 107 pitches over 5 2/3 innings. The right-hander walked two and struck out five.
"For the most part, it's a team thing," said Buchholz. "If the team loses, nobody really cares about how the starting pitcher did."
In this one, it all came down to the bullpen. And this was just one of those nights where Francona couldn't find relief from anywhere.
For the young A's, it was a night to relish.
"The odds of pulling that off are slim, and we did it," said Oakland manager Bob Geren. "The one thing you learn is that you keep fighting until the end."