Riggleman noticed his team was too quiet in the dugout, didn't ask questions when things went wrong and had problems hitting right-hander John Maine, who pitched five shutout innings and gave up two hits. In fact, Maine had a no-hitter until Josh Willingham singled to lead off the fifth inning.
The Nationals didn't score any runs until the ninth inning, when Ryan Zimmerman and Adam Dunn had RBI hits against reliever Brian Stokes, but the score was out of reach by the time they did their damage.
"It was just a lackluster performance. We can't have that," Riggleman said. "We were flat, and it's just not acceptable. We were down a run and it was almost like, 'Let's see what we can do here to get through it.' I just wasn't pleased with the overall intensity of the game.
"The thing is, [Maine] is throwing a good ballgame. Good pitching will make you look flat, and he pitched pretty good. We can't allow that to happen.
"We have a lot of guys who are fighting on this ballclub to make an impression for the future. I just reminded them that the last couple of weeks of the season count. You can't play with a lack of energy. If you do, it's going to show up in somebody's mind, and [that person] is going to be making decisions about your future in this organization."
Dunn disagreed with Riggleman's assessment about Sunday's game, saying there was energy on the field. Dunn also believes that it's tough to generate enthusiasm if a team is losing almost every single day.
"I was in the game, so I'm not going to sit here and say I was flat. I wasn't, and I can only speak for myself," Dunn said. "I'm going to say no, I don't think we were flat. We ran against some guys that pitched pretty good. We are not swinging the bats very well. That's been the case."
Asked if it was tough to maintain the energy against a pitcher like Maine, Dunn said: "It's beautiful outside. It's easy [to have energy]. Good weather. What else are you going to do? We played X amount of games, why would we decide [on Sunday] to not play hard? I don't care how we are winning or losing, we do play hard. The energy was the same."
Right-hander Garrett Mock, who started for the Nationals, is becoming a creature of habit. On Sunday, he got off to another slow start on the mound and it proved costly again.
Mock lasted seven innings and gave up four runs on nine hits. Like his previous start against the Phillies last week, Mock kept the ball up in the early going. In the second inning, after Daniel Murphy tripled, Josh Thole gave New York a 1-0 lead with an infield single.
In the next inning, Carlos Beltran singled home Angel Pagan, while Murphy drove in two more runs with a double. Mock would go on to blank the Mets for the next four innings.
Riggleman said if the team was in a pennant race, Mock would have been out of the game earlier.
"Mock gave us four good innings after the damage was done. If that is a meaningful game, we might have to pinch-hit for him in the future," Riggleman said. "You put your club behind -- four runs in three innings -- you don't get those opportunities to pitch those next four innings and show how good you are.
"We are able to be patient and stretch him out. You can't test the manager that way with those games where a pitcher is down three or four early."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.