Despite a quality start from starter Edwin Jackson, the Nationals lost their third straight game. (Alex Brandon/Associated Press)

The first-place New York Yankees concluded a three-game sweep of the Washington Nationals on Sunday at Nationals Park. Washington’s offense was stagnant. The young team made mistakes. Three winnable games resulted in three losses. But with the team still leading the National League East, none of that is cause for alarm just yet.

What was eating Nationals Manager Davey Johnson after his team’s 4-1, series-closing loss was the state of his cornerstone third baseman, Ryan Zimmerman, and his continued struggles at the plate. Zimmerman’s 0-for-4 performance dropped his batting average to .229.

“That really concerns me,” Johnson said after the game. “We’ve got to get Zim going. Need to get Zim in a happy place.”

The Nationals played competitive baseball throughout the series despite cold bats, a handful of defensive mistakes and two poor performances by reliever Brad Lidge that led to him being designated for assignment on Sunday. They capably handled the pressure of a showcase series against the Yankees, who have the second-best record in baseball after their ninth consecutive win. The young Nationals can learn from this series.

“It’s about as close to a playoff atmosphere as you’re going to get,” first baseman Adam LaRoche said.

But Johnson has been worried about Zimmerman since the third baseman came off the disabled list on May 8 following a shoulder injury. For so long, when the Nationals were at their worst, Zimmerman was at his best. He is an all-star player and leader capable of hitting 30 home runs, driving in 100 runs and playing Gold Glove defense at a demanding position.

The defense is still there. The leadership, too. The offense isn’t.

Before Sunday, Johnson had repeatedly said Zimmerman is too good of an athlete to continue struggling and that he wasn’t concerned about the slump because of his player’s track record. After Sunday’s game, Johnson wondered if the shoulder injury that caused Zimmerman to miss 13 games was still bothering him. But Zimmerman insisted he felt fine.

While the Nationals’ offensive struggles this series weren’t solely Zimmerman’s fault, he offered no help. He was 1 for 13 in the series with two strikeouts, one walk and no RBI.

On Sunday, Zimmerman was at the plate for his second at-bat with two outs in the third inning with rookie Bryce Harper on second following a double. Zimmerman took right-handed starter Ivan Nova’s first pitch for a strike and fouled off the second offering. But on the third pitch, a curveball, Zimmerman swung and missed.

The agony would continue for the Nationals’ $100 million player. He grounded out to shortstop Derek Jeter to lead off the sixth and flied out to right field with Harper on first base and one out in the eighth inning. Overall, Zimmerman saw only seven pitches on Sunday. Two of his at-bats lasted one pitch.

“I’m not doing as well as I want to,” Zimmerman said. “I think anytime you’re not getting hits like that something has to change. I’ve just got to keep working hard and keep doing the same things I’ve done my whole career and it’ll turn around. The worst thing you can do is panic and try to change everything.”

The Nationals (38-26) maintain a four-game lead in the NL East despite the struggles of the middle of their lineup. In addition to Zimmerman, slugger Michael Morse has a .211 batting average in 14 games since returning from injury on June 2.

Nova was challenged little by the Nationals, who put many of his first pitches into play. He left the game with two outs in the eighth inning having thrown only 97 pitches. Except for LaRoche’s solo home run in the second inning and his double in the fourth, and for Harper’s double in the third, the Nationals didn’t drive the ball well.

LaRoche’s leadoff double in the fourth gave the Nationals a chance to break a 1-1 tie. But Morse struck out and, after Ian Desmond singled to move LaRoche to third, Danny Espinosa (.230 batting average) grounded into a double play to end the threat. The Nationals went 1 for 6 with runners in scoring position on Sunday and 4 for 24 in the series.

“I want us to be aggressive, and sometimes the middle of the order is too passive,” Johnson said. “We’re not striking any fear in the opposing pitcher. They’re having it too easy with us. And those guys need to get going. They’re certainly talented enough to get going. Whatever we’ve got to do to get it going, the whole season’s in front of us.”

Johnson said he hoped to meet with Zimmerman sometime this week to help sort out what’s been bothering him.

“It’s just a matter of working on [my swing] and taking it to the game,” Zimmerman said. “And you know, just having success there. You have a couple good games in a row and you get going, and it snowballs and you get on track.”