It didn't help that the Nationals couldn't adjust to McDonald's curveball, at which they were swinging and missing most of the time.
"That curveball, we weren't getting on it for whatever reason," LaRoche said. "We couldn't figure it out until late. By the time we did, in that one inning, he was out of the game. Too little, too late."
The Pirates were able to handle Zimmermann, who allowed four runs in six innings. Manager Davey Johnson felt Zimmermann had his worst outing of the season.
"Well, that was a tough one," Johnson said about Zimmermann. "He was rushing. That's the worst I've seen him with his command early in the ballgame. He got better in the fifth and sixth inning. But early on, he threw almost 20 pitches an inning. That is not typical Zimm."
Zimmermann didn't contradict what his manager said about his outing.
"I didn't feel very good at all," Zimmermann said. "I was flying open and I wasn't able to locate my fastball at all. The breaking stuff was good, but when you can't locate your fastball, you are going to be in trouble. It's going to be a long game."
Andrew McCutchen was the dominating figure in the scoring, hitting two solo home runs. During his career, McCutchen has owned the Nationals, hitting .442 with 10 homers in 23 games.
"He has quick hands and he can pretty much get to any pitch," Zimmermann said about McCutchen. "I think we have to bust him in next time we face him. I think he was sitting on that slider. And the fastball that he hit in the first inning was a little up. He drove it the other way. He is a good player, and you have to make your pitches."
But the Nationals were able to get to McDonald in the sixth inning, scoring three runs. With runners on second and third and two outs, Ryan Zimmerman singled to right field, scoring Jesus Flores -- who got the first hit off McDonald, a double -- and Steve Lombardozzi.
LaRoche was the next hitter, and he tripled over Jose Tabata's head in left field, scoring Zimmerman.
The Pirates made it a two-run game in the top of the seventh inning against reliever Craig Stammen, when Josh Harrison scored on a groundout by Neil Walker.
The Nationals had a chance to at least tie the game in the bottom of the seventh inning, with Juan Cruz on the mound. They had runners on second and third with one out, but Lombardozzi grounded out to Cruz and Ian Desmond struck out to end the inning.
"We were knocking on the door the whole time," said Desmond, who struck out four times in the game. "I felt like we had opportunities. I had opportunities, and kind of squandered them. We were right there. We could have won that ballgame very easily."
Washington had the tying run on base against closer Joel Hanrahan in the ninth, but he managed to save his eighth game of the season.
With runners on first and second and one out, Lombardozzi popped up to third baseman Pedro Alvarez, and Desmond then struck out to end the game.
"The positive is that we had opportunities," LaRoche said. "You are not going to always capitalize on them. The fact that we had guys in position and a chance to win it a couple of times, that's all we can do."