BOSTON – Booed off the field as he made yet another trip to the pitchers’ mound, Bobby Valentine searched for something encouraging to take out of Boston’s fifth consecutive loss.

“I think we’ve hit bottom,” the Red Sox manager said Saturday after his bullpen blew a nine-run lead, allowing back-to-back seven-run innings to hand the New York Yankees a 15-9 victory.

“If this isn’t bottom, we need to find some new ends of the earth.”

Nick Swisher hit a grand slam in the seventh to help the Yankees erase a 9-0 deficit, then added a two-run double in the eighth to give them the lead. Mark Teixeira had six RBI, homering from both sides of the plate for the 13th time in his career.

“That was pretty cool,” Teixeira said. “I’ve played a lot of games and that may be the most fun regular-season game I’ve ever been a part of.”

A day after Red Sox starter Clay Buchholz gave up five homers and the Yankees spoiled Fenway Park’s 100th anniversary party, Boston chased New York starter Freddy Garcia after 12/3 innings and opened a 9-0 lead through five. But Teixeira homered twice, the second a three-run shot in the seven-run seventh that cut the deficit to 9-8.

Swisher, who also had six RBI, hit a two-run double off the center-field wall in the eighth to make it 11-9. The Yankees added five more runs as Valentine used three pitchers to get out of the inning. Closer Alfredo Aceves (0-1) came in to protect a 9-8 lead with nobody out and allowed five runs while failing to record an out.

“I don’t like to lose. I don’t know anybody who does,” Boston shortstop Mike Aviles said. “This wasn’t fun at all. I don’t want to see it if it gets any worse.”

Rafael Soriano (2-0) pitched one inning of scoreless relief.

It was Boston’s second straight loss to the rival Yankees, each of them heartbreaking in its own way. Friday’s 6-2 loss cast a pall over the feel-good 100th anniversary celebration, but the sequel Saturday exposed the bullpen and had the hometown fans booing Valentine just 14 games into his Red Sox career.

Valentine tipped his cap to the fans as they booed him when he walked back from making a pitching change in the eighth.

“I’ve been booed in a couple of countries; a few different stadiums. I don’t want to be booed,” said Valentine, who was asked if he had any regrets about leaving broadcasting to take the Red Sox job. “If they said it was only going to be for the good days, I probably wouldn’t have come. The challenges are great.”

Felix Doubront held New York scoreless on three hits until Teixeira hit a solo homer with two outs in the sixth. By that time Boston led 9-0, thanks in part to a two-run homer by Cody Ross, and RBI doubles from Adrian Gonzalez and David Ortiz.

Ortiz had four hits and a walk, and Aviles drove in a pair of runs for Boston.

The Yankees improved to 9-3 since they were swept in the season-opening series by the Tampa Bay Rays.

The Red Sox scored two in the first on back-to-back RBI doubles from Ortiz and Gonzalez, added three in the second to chase Garcia, then made it 7-0 in the third.

Ross homered in the fifth to give Boston a 9-0 lead.