Braden, who has been ailing for the past few days -- Geren unsure until just before game time if Braden would be able to take the mound -- lasted just 3 1/3 innings, giving up four runs on six hits.
"I feel all right, just a little weak, nothing major," said Braden, who fell to 2-2, with a 5.10 ERA. "Probably just some bad pizza. Who knows?
"I felt all right. I got into some jams, got out of some jams, didn't get out of one. I felt fine, all things considered, just tried to give my team a chance."
The Sox wasted little time opening the scoring, as leadoff hitter Dustin Pedroia doubled to center field and scored when the next batter, Kevin Youkilis, singled to center in the first inning.
The jam Braden couldn't get out of snared him in the fourth inning, as Mike Lowell, who doubled to center with one out, and Jason Bay, who singled to left, scored on Jed Lowrie's triple over the head of center fielder Carlos Gonzalez, knocking Braden from the game.
Santiago Casilla entered, walking Jason Varitek and giving up a sacrifice fly to Coco Crisp, which scored Lowrie to put the Sox ahead, 4-0. Casilla then got Jacoby Ellsbury to ground out to Jack Hannahan at third, ending the inning.
"I guess that is my job to determine whether or not I can pitch or not," Braden said. "But if you're going to give me the ball in Boston, if you're going to give me the ball in Fenway, I'm going to take it every time. You need to take my left arm before you can tell me I can't go out there on any day. But just kind of went with it, tried to do everything I can to prepare myself.
"If it would have been detrimental to my performance, I don't think I would have gone out there. I felt like I could compete. I liked the fact that the manager had confidence in me to go out there and compete. I tried to do everything I can to convince him that I was ready to go, and he felt the same way."
The A's threatened in the second when Gonzalez started the frame with a triple to center off Sox right-hander Daisuke Matsuzaka. But Gonzalez could not advance, with Jack Hannahan striking out after a 10-pitch at-bat, Bobby Crosby fanning and Daric Barton flying out to center.
The A's cut the deficit in half in the next inning when Barton sent the first pitch from Matsuzaka into Fenway's right-field seats, scoring Crosby, who had walked. It was Barton's fourth round-tripper of the season, and first since returning from the disabled list with a strained neck on Friday.
One of the points Barton worked on while in rehab with Triple-A Sacramento was being more aggressive at the plate.
"Early in the count, yeah, I'm just trying to look for a ball in the middle of the plate," Barton said. "I have a game plan up there, and if they throw something to my game plan, then yeah, I'm going to be aggressive and swing more aggressive than I have been. Lately, I've been real tentative, and that was the main thing I worked on down there. And that one pitch, I got a ball over the plate early in the count, and I swung and got it."
The bullpen combined for 4 2/3 innings, giving up one unearned run on two hits and two walks, striking out three.
"They've been strong most of the year and again today," Geren said. "I think they're all fighting down there from some quality innings, and it's good. Competition is good."
As thunder and lightning threatened from the west, Joey Devine relieved Casilla to start the sixth inning, giving up a single to Bay and walking Lowrie, who both advanced on a passed ball by catcher Rob Bowen. With a full count to Varitek, the next batter, a brief downpour halted the game at 12:45 p.m. PT.
When play resumed 37 minutes later, Alan Embree took over on the mound for Devine, notching a rare one-pitch strikeout on Varitek, a switch-hitter who started the at-bat as a lefty and ended it as a righty. But a Crisp sacrifice fly to Gonzalez scored Bay, extending the Sox lead to 5-2.
Despite the lost weekend at Fenway, Geren was able to find a silver lining in the storm clouds.
"You look, bad series, going 0-3," Geren said, "but we did hit a few home runs here, which we've been lacking. And our bullpen's been [giving] just stellar performances, basically real good. You've got to try to always take something positive out of a loss."