"I found out I was in the lineup right away, it gave me a couple of hours to prepare. I made sure I was ready," Hairston said. "I've played in this role for the last few years. It's a very difficult role. I rely on that experience."
Daniel Murphy also went deep and Jonathon Niese pitched seven strong innings to help the depleted Mets snap a three-game skid. Fielding a makeshift lineup minus its four biggest bats, New York rapped out 15 hits and improved to 4-7 this season against the NL East leaders, who own baseball's best record.
Beltran sat out with the flu and a high fever, so manager Terry Collins inserted a not-so-secret weapon against Hamels (11-5).
Hairston batted third in large part because he entered with a .353 career average (6 for 17) and three homers against the left-hander, one of Philadelphia's three All-Star starters.
"When you have good numbers against an opponent, it gives you confidence. Confidence helps at the plate," Collins said.
Those big numbers quickly ballooned even more.
Hairston doubled his first two times up, driving in two runs and scoring one as New York built a 4-0 lead by the fourth. He hit a three-run shot off Danys Baez in the seventh to make it 11-2, a drive to left that sailed into the rarely reached second deck at spacious Citi Field.
New York totaled 10 runs in its previous five games, an understandable slump considering the competition. Hamels was the fifth All-Star starter the Mets faced in a span of six games, going 2-3.
"I just wasn't hitting my spots," Hamels said. "They're a good team. Obviously they're young, but at the same time you have to make pitches and I wasn't able to make pitches."
Niese (9-7) allowed one earned run and six hits while striking out six and walking one.
The left-hander, who leads the team in wins and is 9-4 in his last 15 starts, hardly ran into any trouble besides a bases-loaded jam in the seventh that he soon escaped.
"He is getting tougher," Collins said. "It's as if he's saying we're going to get back on track and I'm going to take care of it."
Murphy, who had three hits and two RBIs, opened the fifth with his sixth home run to make it 5-0 -- and the Mets didn't stop there.
Jason Bay singled and, one out later, lumbering Nick Evans hit a long drive to right for his second major league triple. Ruben Tejada grounded reliever David Herndon's first pitch through a drawn-in infield for a 7-0 cushion.
Hamels yielded seven earned runs, matching a career high, and eight hits in 4 1/3 innings while falling to 3-10 in 16 career games against New York.
He lasted only 2 2/3 innings in a 7-1 loss to the Mets on April 5 and had dropped five consecutive starts against them before striking out 10 in a 5-2 victory May 28 at Citi Field.
"Haven't really been able to get a good roll against them," Hamels said. "It's kind of one of those games where you just want to throw it out."
His demise Saturday came much to the delight of fans who remember that Hamels called the Mets "choke artists" in December 2008 after their second consecutive September collapse.
Miscommunication on the Philadelphia infield made for an unlucky start for Hamels, whose ERA spiked from 2.32 to 2.71.
Hairston doubled with two outs in the first and hustled home when Murphy's harmless-looking popup dropped between first baseman Ryan Howard and second baseman Chase Utley, who both backed away as the ball descended.
"Originally I called it and then it was kind of loud and I thought he said that he had it," said Howard, who sat staring into his locker, still in uniform, for a long while before hitting the showers. "I kind of saw him in the corner of my eye and heard him. He was saying my name and I mistook it for him saying that he got it, that he was calling me off."
Howard got a derisive cheer when he gloved Evans' foulout the next inning.
Phillies CF Shane Victorino (sprained right thumb) was set to begin a rehab assignment Saturday night with Double-A Reading at Trenton. Philadelphia reliever Brad Lidge (strained rotator cuff) also was scheduled to appear in that game. ... Plate umpire Brian Gorman was hit on the left hand by a pitch from Hamels that bounced past Phillies C Carlos Ruiz. Gorman said X-rays were negative and the swelling had already decreased dramatically thanks to an anti-inflammatory patch applied by the Mets' training staff. "I'll live," he said. ... Beltran was not at the ballpark and his status for Sunday was uncertain, Collins said. ... Pagan snapped an 0-for-17 slide with an infield single in the fifth. ... Hairston has four doubles off Hamels in 20 at-bats. ... Attendance was 41,166.