BOSTON — An 11-game road trip that brought the possibility of pushing the Yankees further into the thick of the American League playoff race already is threatening to completely take them out of it.

After suffering what was hands-down their worst loss of the season Thursday night, the Yankees dropped one in a far more conventional manner Friday night, falling to the Red Sox, 7-4, in front of 37,927 at Fenway Park.

The Yankees (77-70), who have lost five of their last six, are six games behind the AL East-leading Red Sox and four games behind the Orioles and Blue Jays, who are tied atop the race for the two wild cards.

Does it feel as if the Yankees’ playoff hopes are slipping away? “Nope, not to me,’’ Chase Headley said. “Obviously, we would have liked to win a couple of games here and could have gained a little bit of ground, but there’s still some time if we come out and play well. It’s reachable. We just have to go and get on another streak and start winning some games.”

Rookie righthander Luis Cessa (4-2) wasn’t awful, allowing three runs and six hits in five innings. The bullpen, brought on to try to hold a 3-2 deficit, allowed four runs that made it 7-2 before pinch hitter Billy Butler hit a two-run homer in the top of the ninth.

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Joe Girardi said of pulling Cessa after 64 pitches: “Just thought we’d start mixing and matching at that point. We thought his fastball was starting to leak a little bit, so we decided to make a change.”

Through a translator, Cessa said, “I was a little surprised, but at the end of the day, it’s not my decision.”

Boston pushed the lead to three runs in the sixth. David Ortiz doubled off James Pazos and Mookie Betts singled against Jonathan Holder. Hanley Ramirez walked to load the bases and Travis Shaw’s sacrifice fly to center made it 4-2. Holder struck out Chris Young looking, but Sandy Leon blooped a 1-and-2 pitch down the leftfield line for a ground-rule double that made it 5-2.

Jackie Bradley’s Jr. 25th homer, on Chasen Shreve’s first pitch of the seventh, produced a 6-2 lead.

“These guys have to step up,’’ Girardi said of turning close games over to young relievers. “That’s what we’re asking them to do.”

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Boston righthander Clay Buchholz (7-10, 5.20) allowed two runs and seven hits in six innings.

The Red Sox, who outhit the Yankees 12-9, gave Buchholz a quick 2-0 lead with a strange bottom of the first, an inning in which Dustin Pedroia and Ortiz were thrown out trying to stretch singles into doubles and Betts apparently was thrown out at second on an attempted steal before the call was overturned.

Ortiz drove in the inning’s first run and Ramirez — who won Thursday night’s game with a walk-off three-run homer to centerfield off Dellin Betances with two outs in the ninth — drove in the second with a two-out single. Ramirez made it 3-0 in the fourth with his 26th homer, another blast over the centerfield wall. It was the 14th homer allowed by Cessa, who has pitched 52 2⁄3 innings.

Mark Teixeira and Mason Williams began the fifth with singles, but Brett Gardner struck out and Jacoby Ellsbury flied to right. Gary Sanchez, however, launched one off the Green Monster in left-center for a two-run double that made it 3-2.

Butler’s homer in the ninth gave him four RBIs in two games, but the Yankees went 1- for-11 with runners in scoring position and stranded eight. They left 12 on base Thursday night.

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“Things could change quickly,” Girardi said of the dire position his team finds itself in. “We saw it happen. We saw it change in a week dramatically and it could happen again.”