Red Sox outfielder Jason Bay connects for a single in the fifth inning last night against New York at Yankee Stadium.

AP / Frank Franklin II

NEW YORK — Justin Masterson experienced the biggest moment of his young major-league career last night in Boston’s 7-3 victory.

The Red Sox’ rookie right-handed reliever entered the game with a four-run lead and one out in the bottom of the seventh inning. Might not seem like much, but here’s the kicker. The bases were loaded — thanks to fellow reliever Manny Delcarmen — when Masterson was brought in to face the Yankees’ Alex Rodriguez.

Masterson, only 23 years old, needed just two pitches to do exactly what he was brought in to do. His first offering was a 91-mph sinker for a called strike. His second pitch — another sinker — was down enough in the zone to get Rodriguez to ground into a double play.

The threat was over.

“That’s obviously the biggest pitch of the game,” said Red Sox manager Terry Francona. “We got ourselves into a position where one pitch could turn the game around, and he makes the one pitch to get us out of it.”

Some might think that a bases-loaded situation would be too much of a pressure-packed situation for a rookie pitcher in the atmosphere that is Yankee Stadium. Not for Masterson.

“I love that stuff,” he said. “That’s why you play this game. I was just happy to help us get a win.”

Masterson made a lifetime memory. Rodriguez made 55,058 fans boo him.

“I was just doing my job,” added Masterson. “He’s a good right-handed hitter and when I’m called upon I’m supposed to get the job done. I won that time and I’m excited to keep going.”

It wasn’t the first time Masterson had toed the rubber here. He made a start at Yankee Stadium on July 5, and even though he suffered a 2-1 loss, he was solid during his six-inning outing, in which he allowed the two runs on six hits.

That day Masterson faced Rodriguez three times. The Yankee third baseman grounded out, was hit by a pitch and had a single in the three at-bats. No question it was that earlier experience that helped Masterson win the battle last night.

“He’s been huge for us this year,” said Red Sox catcher Kevin Cash, who worked as Masterson’s batterymate last night. “He came up as a starter and he gave us a chance to win every time. Then they make a decision to put him in the bullpen to help the club, and I would say [last night] was his biggest moment, as far as coming in with the bases loaded against arguably the best hitter in the game. He made his two pitches. He’s not a very emotional guy, but he was pumped.”

The Yankees’ Johnny Damon gave New York an early lead with a solo homer in the bottom of the first inning. He crushed a 1-2 offering from Tim Wakefield and deposited it into the right-field seats for his third lead-off homer of the season.

Boston got to New York starter Andy Pettitte in the top of the second inning with two outs when the club’s No. 8 and No. 9 hitters, Jeff Bailey and Kevin Cash, reached on back-to-back infield hits. Jacoby Ellsbury’s RBI single drove in Bailey to tie the game at 1-1.

New York regained its lead in the bottom of the inning on Jose Molina’s two-out single before Boston took the lead for good. The Red Sox led off the third with back-to-back doubles by David Ortiz and Kevin Youkilis. Ortiz tied the game at 2-2 when he scored on Youkilis’ double to left, and the Red Sox jumped ahead, 3-2, on Jason Bay’s RBI single.

Boston added to its lead in the fifth inning when it pushed across three runs for a 6-2 advantage. With two outs, Bay and Jed Lowrie provided back-to-back singles before Crisp’s RBI single. Bailey then continued his interesting night when his second infield hit drove in two to give Boston its three-run lead.

Damon hit his second homer of the game — his 11th multi-homer game of his career — in the bottom of the fifth inning to cut New York’s deficit to three, 6-3. Bay’s sacrifice fly in the sixth inning gave Boston its 7-3 lead.

After reliever Javier Lopez worked a perfect 1-2-3 sixth inning, Delcarmen struggled in the seventh when he loaded the bases and needed Masterson to get out of it. Hideki Okajima and Jonathan Papelbon finished the job.

“I got the first out again,” said Delcarmen. “Then I was down in the zone and just missing. I’m just glad he got that ground ball. We saw a lot of that [guys picking each other up] last year. Hopefully we won’t keep throwing ourselves into those situations, but he picked me up big-time tonight.”

“That’s what the bullpen’s supposed to do,” Masterson said. “I’m just a small piece.”

At 9:37 last night in New York, Masterson had his moment. More importantly, the Red Sox finished with the victory.

In his first start back after a stint on the disabled list, Red Sox veteran knuckleballer Tim Wakefield had one word to describe the rookie’s performance.

“Phenomenal,” he said.

jmcdonal@projo.com