BOSTON -- Dustin Pedroia was trying to be the face of confidence standing in the middle of a very quiet Red Sox clubhouse.
"We've got the lead. We've just got to go out and play better. We're not hanging our heads," he said after fading Boston lost for the 12th time in 15 games, dropping the opener of a day-night doubleheader 6-5 to the Baltimore Orioles on Monday.
Jeremy Guthrie (9-17) pitched the last-place Orioles to the win, cutting Boston's once-large AL wild-card lead to 1½ games over charging Tampa Bay.
Boston rallied from a 6-2 deficit and cut the lead to two when a missed call cost the Red Sox a run and halted their comeback bid.
TV replays appeared to show that David Ortiz's fifth-inning liner down the right-field line -- ruled foul by first-base umpire Mike Estabrook -- seemed to carom fair off the lower part of a short wall near the Pesky Pole with Pedroia on third after an RBI triple.
"That's the breaks of the game," Pedroia said. "We've played a hundred-something games. We're not going to say the season's over because an umpire missed a call."
Boston, which lost three of four games against Tampa Bay this past weekend, led the Rays by nine on Sept 3. The Red Sox have dropped 15 of 20 since they held a 1½-game edge over New York atop the AL East on Aug. 30.
The Yankees now lead the division by 5½ games.
"Any loss hurts," Boston manager Terry Francona said. "We battled, and battled and battled and came up short. I'm glad we're playing again right away."
The Red Sox have six games remaining against Baltimore and three in New York against the Yankees. The Rays have 10 to play.
Baltimore won for the first time in six games at Fenway Park this year, getting outscored 40-20 in the five loses.
"These are some pressure games. I want to see those guys in those situations," Orioles manager Buck Showalter said. "Hopefully down the road they'll be pitching in games that mean a little bit more for us."
Guthrie pitched into the seventh inning for his first career win in Fenway Park. He had been 0-3 and the Orioles had lost all seven of his previous starts.
Guthrie was pulled after McDonald's leadoff single in the seventh. He allowed four runs and 10 hits.
"It's really nice," Guthrie said. "Probably one of my favorite days as a baseball player."
Jim Johnson, the sixth Orioles pitcher, worked a perfect ninth for his seventh save.
Leading 2-1 in the fourth, Baltimore roughed up Kyle Weiland (0-3) when Andino and Reimold hit consecutive homers into the Green Monster seats.
The Red Sox trailed 6-2 in the fifth before they mounted a comeback.
Adrian Gonzalez had an RBI double and Pedroia followed with a triple off the center-field wall, cutting it to 6-4 against Guthrie, before Ortiz's drive was ruled foul.
"It's huge," Red Sox catcher Jarrod Saltalamacchia said. "Like I said, the game's about momentum."
After the umpires huddled in the middle of the diamond, the call was upheld. Francona argued along with Ortiz and first base coach Ron Johnson. Ortiz then flied to the track in center, pounding his helmet to the ground in frustration.
"I wish they could have kept asking somebody else, but they ran out of people to ask," Francona said.
The Orioles took a 2-0 lead in the third when McDonald, inserted into the lineup when left fielder Carl Crawford was scratched with a stiff neck, lost Reimold's fly ball in the sun. The ball ticked off his glove for a single, and McDonald then dropped Josh Bell's for a two-base error before Matt Angle hit a two-run double over his leaping try against the wall.
McDonald, who came up to spattering of boos leading off in the bottom of the inning, homered into the Monster seats.
Weiland, making his fourth career start and his third against the Orioles, lasted just 4 2/3 innings, allowing six runs and five hits.
Showalter said he wasn't ready to put INF Mark Reynolds back in the lineup after he missed Sunday's game. Reynolds was hit in the helmet by a pitch on Saturday. "I'm not in a position where we feel comfortable running him out there. We'll see what (Tuesday) brings," he said. ... Francona said OF J.D. Drew may see a doctor soon to check the "stability" of a broken middle finger and if all is well they may "move forward a little bit" toward his return. Drew has been sidelined since July 20 with a left shoulder injury and broke his finger on a rehab assignment. ... Francona also said RHP Dan Wheeler has been bothered by forearm stiffness. ... Showalter, critical in a magazine article during spring training of Red Sox GM Theo Epstein's ability to use a large payroll, wouldn't say if beating the Red Sox meant more now. "You don't ever think about that," he said. "We're just trying to take care of our business. I'd like to be in their shoes. We're trying one day to figure out how to be as good as they are." ... It was a makeup of a May 17 rainout. ... John Lackey (12-12, 6.19 ERA) goes for Boston against struggling lefty Brian Matusz (1-7, 9.84 ERA) in the nightcap.