The Mets’ lineup has been slumping for a week, yesterday’s starter Shaun Marcum struggled in his debut and reliever Robert Carson was even worse. By the time it was over, the Mets had suffered a 9-4 beating at the hands of the Phillies and were mired in their first three-game skid of the year.

Marcum (0-1) lasted just four innings in his injury-delayed Mets debut, giving up three earned runs and Carson coughed up five more — including back-to-back homers — in his only inning and was booed lustily by the 29,248 at Citi Field, who saw the Mets fall two games under .500 for the first time this season.

“That’s not what I wanted from a statistical standpoint or helping the team. I wasn’t very efficient with my pitches in the second or third innings,’’ said Marcum. “I’ve got to ... make sure next time I’m putting the ball in play a lot earlier in the count and go deeper into the game. ... I’ve got to get a lot more efficient, locate and keep the ball down.’’

The Mets demoted Josh Edgin to Double-A yesterday to make room for Marcum, who made his first start after neck and shoulder woes in spring training. But he couldn’t minimize the damage in a three-run third, leaving for Carson an inning later down 3-2, and the bullpen imploded again.

“It’s not like we’re playing terrible baseball. It’s one or two innings that are hurting us,’’ said catcher John Buck, who hit his eighth homer, a solo shot in the ninth. “If we were going out there not putting together good at-bats, or continuously having bad pitching, maybe [I’d worry]. But that’s not the case. It’s the one big inning that’s hurting us.’’

It hurt them again. Lefty Scott Rice was roughed up in the ninth inning Friday, and yesterday Carson, the struggling bullpen’s other southpaw, served up a three-run homer to Domonic Brown and a solo shot to John Mayberry Jr. just a pitch later for an 8-2 hole.