Nationals right-hander Jordan Zimmermann allows two hits against the Mets, but one is a home run by John Buck, left, rounding the bases after his solo shot in the second inning. (Ray Stubblebine/REUTERS)

The puzzling beginning to the Washington Nationals’ season continued Sunday afternoon, a mix of inattentive play, impotent bats and shoddy defense topped with one pivotal, dubious swing. The root cause may be their pressing against the weight of expectation, regression from last season or — the Nationals’ chosen explanation — simply a slow start that will be forgotten by July. Whatever the reason, the Nationals at this moment look like a team aspiring to play fundamental baseball, not to win the World Series.

In a 2-0 loss to the New York Mets at Citi Field, the Nationals overshadowed Anthony Rendon’s hitless major league debut with their slackness. The Nationals swatted only four hits as they suffered their third shutout of the season. They committed three errors, which pushed their league-worst total to 18 in 18 games. They squandered their best, last rally in the eighth inning when Jayson Werth swung away on a 3-0 pitch and grounded into a double play.

The Nationals will return to Washington on Monday for a series opener against the St. Louis Cardinals with a chance to establish themselves against the opponent who ended their 2012 season. The Nationals, at 10-8, are on pace to win 90 games. But they also know they have not scratched their potential and understand they must improve to make it back to October.

“If we continue to take the same approach that worked and the same thing we did last year, it should even out,” first baseman Adam LaRoche said. “But, yeah, this is unacceptable right now. There’s some pretty bad baseball.”

After blundering their way through the afternoon, the Nationals gave themselves a chance to steal a victory from the Mets. They trailed only 2-0 in the eighth because Jordan Zimmermann and Zach Duke had weathered errors from Ian Desmond, Rendon and Bryce Harper as the Nationals’ bats wilted against starter Dillon Gee, a right-hander who entered with an 8.36 ERA.

The Post Sports Live crew discusses whether the Nationals bullpen woes, Ryan Zimmerman’s throwing errors and Wilson Ramos’s injury are reasons to worry. (Post Sports Live)

They had tapped into the soft belly of the Mets’ bullpen, represented in the form of Scott Rice, a left-hander who spent 14 years in the minors before this year. Steve Lombardozzi smacked a pinch-hit single. Denard Span walked. Werth took three consecutive balls for a 3-0 count.

Werth moved up in the batter’s box, looking for a pitch he could pull and drive over the fence. Rice had thrown seven straight balls, but “I felt like he was going to groove one,” Werth said. “I felt I could do damage.”

Werth, the oldest and most experienced player on the Nationals, instead achieved a measure of ignominy. He became the first big leaguer of 2013 to ground into a double play on a 3-0 pitch. He hacked at an outside, 88-mph sinker and rolled it to shortstop. Ruben Tejada flipped to David Murphy, who fired to Ike Davis.

“I was caught up in the moment,” Werth said afterward. “Looking back, I was trying to do too much. I was trying to win the game right there. That’s it, really. We can sit here and talk about the situation. I was just trying to do too much. The situation got the best of me. That’s probably one of the dumber things I’ve done on the field in a while. Look no further than right here. We had a chance to win the game. I feel like I pretty much blew it.”

There was logic in Werth’s willingness to swing — two left-handers, Harper and LaRoche, would hit behind him, and so Werth may have had the best chance to belt a game-changing hit. But there was little defense for swinging at this particular pitch. “I couldn’t even tell you if it was a strike,” Werth later admitted. Rice was trying to aim down the middle and let the sinker’s movement take it to the edge of the plate.

“We got lucky on that one,” Mets third baseman David Wright said.

When asked about the 3-0 swing, Manager Davey Johnson blurted, “I’m not going to go into that.”

Sunday provided few enjoyable questions for Johnson. Zimmermann and Duke pitched around three Nationals errors as the team’s odd, troubling and season-long defensive negligence continued. Twelve of their errors have been committed by the left side of their infield. Entering Sunday, only seven major league teams had totaled a dozen miscues.

“I have all the confidence in the world in our defense,” Johnson said. “A lot of talented guys out there. Sometimes, you make errors. I saw Brooks Robinson make three errors in one ballgame. Did I worry about it? No. We’re just a little out of sync. We’ll get there.”

Johnson dismissed the notion that the miscues had stacked up because of expectations-induced jitters. The Nationals entered the season favored to win the World Series, and they have performed well below capacity. Whether those facts are related is a conversation piece.

“I hope other guys don’t see it that way, like we’re supposed to be leading the world in every category,” LaRoche said. “It just doesn’t happen. Just because we don’t start out hot doesn’t mean this is the way we’re going to perform all year. It can if you let it get that way, where everybody starts pressing. We’ll look up in a couple months and be in a bad spot.”

The malaise infected even Harper, who typically plays with a fierce edge. In the sixth inning, Mike Baxter ripped a single to left field. Harper charged and then, realizing he had no play, backed off to play the ball on a hop. The ball skipped off his glove and trickled away. Harper looked behind, expecting to see the ball but finding only green grass. By the time Harper located the ball in front of him, Baxter, as Harper has done to so many opposing outfielders, punished him for a split-second of uncertainty. He bolted to second base, and Harper sailed the throw over Danny Espinosa’s head.

“There was nothing I could do,” Harper said. “The good news is, he didn’t score.”

From the beginning, the Nationals made simple plays adventurous. Jordany Valdespin led off the game with a grounder to shortstop Desmond, and he flung the ball past LaRoche at first base. Valdespin took second, and Desmond swallowed his league-leading seventh error.

By the end of the day, the Nationals found themselves in the familiar position of relying on the calendar for solace. It is early, to be sure, but it will only be early for so long.

“We haven’t played very good, in all facets,” Werth said. “Defense hasn’t been right. Offense, we haven’t really lined up. Pitching has been pretty much the same. But we’ll get it going. I couldn’t tell you what our record is, but things could be a lot worse. We’re a good team. We’ll get it together. We’ve got too much talent. We’ll be all right.”