WASHINGTON — The Pittsburgh Pirates weren’t in the mood to celebrate after Andrew McCutchen hit two home runs in a 6-5 win over Washington.

The victory, accomplished Monday night against the skidding Nationals, was meaningless in comparison to the ramifications that could come from a ninth-inning pitch thrown by All-Star closer Jason Grilli.

After the pitch, the right-hander left with forearm tightness — leaving his manager and teammates somberly hoping for the best.

“It’s never good to see somebody in pain, let alone your All-Star closer,” starting pitcher Charlie Morton said.

Grilli entered in the ninth and gave up a two-run homer to Jayson Werth before leaving with an apparent arm injury after getting two outs.

“Pretty much he threw the ball, and his reaction was, he just grimaced a little bit,” Pirates catcher Russell Martin said. “It seemed like he was in a little bit of pain. It just looked like his forearm was bugging him. A definite blow. Hopefully it’s not as severe as it can be and is just something minor.”

Grilli was not made available to the media after the game.

Manager Clint Hurdle tried to remain upbeat, saying, “We’ll wait and see where Grilli’s situation goes. Right now it’s right forearm discomfort. We’re going to wait and see how he feels (Tuesday).”

Asked what the Pirates would do if Grilli was lost for an extended period, Hurdle snapped, “And what if he’s not? Our roster is going to be fine.”

It was a disappointing ending to a night that started so well.

McCutchen connected in the first inning and again in the third to stake the Pirates to a 4-0 lead. The All-Star center fielder has five homers in his last six games and 14 for the season.

Both homers came off Dan Haren (4-11), who’s 0-8 in 11 starts since May 9.

McCutchen is 18 for 35 with five homers and 12 RBIs in his last nine games at Nationals Park.

“Every player always has a ballpark he seems to hit at better than other places,” he said. “I guess mine happens to be here.”

Although he went hitless in his final three at-bats, McCutchen is hitting .431 with 13 home runs and 26 RBIs in 27 career games against Washington.

Morton (2-2) gave up three runs, six hits and a walk in 6 2-3 innings to earn his first win in five starts. He was lifted after yielding a two-run homer to Werth.

“When you get later in the game, especially if you’re pitching well — which I was — you like to finish off the outing on a strong note,” Morton said. “Giving up a two-run homer isn’t exactly what you want.”

But he got the Pirates in position for the win, and after Grilli departed, Vin Mazzaro retired Steve Lombardozzi on a grounder with a runner on second to earn his first career save.

Werth’s two homers gave him four in two games. Adam LaRoche also connected for the Nationals, who have lost four straight and nine of 11 to fall three games under .500 (48-51) for the first time this season.

“There’s a lot of character on this ballclub, there’s no quit, there’s a lot of fight,” manager Davey Johnson said. “By and large everybody swung the bat pretty good.”