It was only one game but it meant the world to the Mets. It would dictate what kind of All-Star break they were going to have. In a way, it also would tell them what kind of nominal first half they had played.

They knew going in that a win would move them within four games of the first-place Nationals, which meant that they had made progress and developed momentum. A loss would mean they would be six games behind and had surrendered the progress they had made previously in the homestand after having been swept in Washington.

The latter scenario was the one that prevailed because the Mets never did draw even after Daniel Murphy’s two-run home run off Steven Matz in the first inning and lost, 3-2. Jose Reyes, who was brought in to help diversify a home-run happy Mets offense, had two home runs. The Mets got nothing else against starter Gio Gonzalez and nothing at all against Tanner Roark — a starter used as a reliever before the four-day hiatus — and closer Jonathan Papelbon.

Murphy started at first base rather than second Sunday, which had absolutely no impact on his concentration and production at the plate. He set a tone for the Nationals with one out and one on in the top of the first. He drilled a 3-and-2 pitch from Steven Matz off the facing of the second deck in rightfield for an instant 2-0 lead.

It was his seventh home run against the Mets this season, and raised his RBI total against his former team to 21.

“Dan Murphy is a student of hitting He studies it, he loves it, he works at it,” Terry Collins said. “What he did in the postseason last year, with some of the changes he made, he obviously said, ‘This works and I’m sticking with it.’ And he has probably even refined it a little bit more. The fact that he’s on the plate, the fact that he’s pulling a little bit more . . . we always knew he had some power but he was one of those kind of guys who would be happy to take a hit to leftfield. Which he still will in certain situations. But when you get him in certain counts where he’s looking to pull, he’s dangerous because he can hit a homer. The last few years, it would be only a hit.

“He knows he’s dangerous enough and what he did last year is carrying over right now,” the manager said.

Murphy’s former club came within 2-1 in the bottom of the first on a blast by another infielder the Mets once let get away. Reyes, finishing the week in which he was reunited with his old friends and fans in Flushing, drilled a pitch from Gio Gonzalez to left-centerfield.

After the Nationals had expanded their lead to 3-1 in the third on a run-scoring single by Wilson Ramos (Matz had earlier retired Murphy on a fielder’s choice), Reyes struck again with a homer down the leftfield line in the third, cutting it to 3-2. It was his fifth hit since returning to the Mets, his third homer.

“If he gets it going, he’s a dynamic offensive player. He can change what we do as a team very, very quickly if he starts swinging the bat like he’s capable of,” Collins said. “We’ve seen it. You know it’s still there, you’ve seen the bat speed. He’s just got to get used to big league pitching on a consistent basis and I think when he does, we’re going to start scoring a lot of runs.”

On this pivotal day, though, they could get no other runs.

Cespedes not ruled out

Terry Collins said before the game he would not rule out using Yoenis Cespedes as a pinch hitter Sunday, even though keeping him out could potentially allow the Mets to backdate a trip to the disabled list. “We certainly won’t have him run very far. I have to worry about winning the game. If it costs me two days on the DL, so be it,” the manager said.

Regardless, the All-Star break is coming at a good time for Cespedes, considering his quad strain. “His rest in the next four days is going to be key to everybody in that clubhouse because we’ve got to come out of the gates in the second half playing real well to get back to right where we want to be like we did last year, and that’s right in the hunt going into those last two months,” Collins said.

Conforto heating up at Triple-A

Michael Conforto went 3-for-4 with a triple and home run for Las Vegas Saturday night. Collins has not spoken with the team’s manager, Wally Backman, in four days so he has no fresh information on the outfielder. The Mets manager acknowledged that there probably will not be room for both Conforto and Brandon Nimmo on the major league roster because the organization wants both of them to play as much as possible.

Goeddels could meet in Philly

The Mets will begin the nominal second half in Philadelphia, which means a prospective meeting of the Goeddels — Erik, the Mets pitcher, and his brother Tyler, a Phillies outfielder. The latter made a brilliant diving catch in a win against the Rockies Friday night. Erik said that Tyler actually is a natural infielder. But, the brother was asked, is Tyler a good outfielder? Erik said, “He is now, I guess.”