Through the season’s first month, John Buck has been the most valuable Mets position player by a wide margin.

Yesterday, however, the Mets’ leader in home runs and RBIs was only valuable to the Phillies.

The veteran catcher’s crucial seventh-inning error kept pinch-hitter Laynce Nix’s at-bat alive, and by the end of the frame, the Phillies had put up a tie-breaking three-spot, and were on their way to a 5-1 victory and a series sweep of the Mets at Citi Field.

“We were out of the inning, we didn’t make a play,” manager Terry Collins said. “At our level, you give good teams extra outs they’re going to do some damage.”

With two outs and nobody on base, Nix popped up a Jon Niese delivery. David Wright and Buck converged in foul territory, near the Phillies dugout, and the ball deflected off the side of Buck’s mitt.

“It hit the glove,” Buck said through clenched teeth. “I should’ve caught it.”

Three pitches later, Nix singled, Jimmy Rollins added a single and Collins called upon reliever Scott Atchison, who yielded a crushing two-run double to Ryan Howard, the Phillies’ second pinch-hitter of the frame.

Collins didn’t say whether he felt Buck should have made the play. He did notice, however, the official scorer called it an error.

“That means somebody thought he should’ve caught it,” Collins said after his team’s fifth loss in six games. “I know it’s a long run, tough play for a catcher. We didn’t make the plays we needed.”

Buck, who also went 0-for-4 at the plate and struck out twice, lowering his average to .250, was waiting for the media after Collins’ press conference.

“I know that as soon as I went into the softer dirt, I felt myself slide a little bit,” Buck said. “It was kind of on the run. I didn’t take my eye off the ball. I just missed it.”