Kevin Frandsen, left, scores ahead of the tag by Pirates reliever Bryan Morris on a wild pitch during the eighth inning. Frandsen was safe on the play, but the Nationals came up short, falling 4-3. (Gene J. Puskar/AP)

At the moment the Washington Nationals mounted their most dangerous threat Friday night, they revealed their most pressing weakness. In the sixth inning, Pittsburgh Pirates right-hander Charlie Morton plunked Ian Desmond with a pitch, loading the bases with no outs. His penance for hitting for the Nationals’ No. 5 hitter would be facing a hitter who didn’t have a team two weeks ago, a rookie who had whiffed in nearly half his at-bats and a veteran whose batting average hovered south of .150.

When, again, do Ryan Zimmerman, Bryce Harper and Adam LaRoche come off the disabled list? The Nationals’ want for their infirm sluggers resonated in their 4-3 loss to the Pirates, a defeat that exposed their thinned lineup and dropped them to .500 for the first time all season. The Nationals have stayed afloat through injuries. After three straight losses, the water is rising.

That sixth-inning rally netted just one run — Greg Dobbs’s sacrifice fly. But the Nationals would not wilt, scratching out two runs off the Pirates’ bullpen in the eighth with a feisty, whatever-it-takes uprising — Kevin Frandsen’s liner to center led to Zach Walters’s bloop single, which led to Scott Hairston’s pinch-hit, bases-loaded sacrifice fly, which led to reliever Bryan Morris’s run-scoring wild pitch.

Afterward, Manager Matt Williams did not outwardly long for his sidelined hitters. Instead, he launched a defense of the players currently on his roster.

“We got Dobbs up there, bases loaded, and he hits a sac fly — got the job done,” Williams said when asked about the Nationals’ missing pieces. “We put Franny up there, he got the job done. Put Hairy in there, he got the job done. They weren’t three-run homers by any stretch, but they’re doing their job. I’m not going to even think about going there, because it’s not fair to the guys that are in here that are busting their tails every day to get this job done.”

Friday night, they didn’t do enough overcome an uneven start from Jordan Zimmermann, who allowed four runs over five innings, the pivotal blow being Pedro Alvarez’s two-run homer.

The Nationals proved they’re willing to fight — plate umpire Todd Tichenor stepped between Danny Espinosa and Pirates catcher Russell Martin after Morris hit Espinosa with a 1-2 pitch and Espinosa and Martin exchanged pleasantries.

It wasn’t enough for the Nationals to win. Anthony Rendon led off the ninth with a walk against Jason Grilli, who had come off the disabled list in the afternoon. Grilli’s save added to the Nationals’ frustration. Desmond ended the game with a smash to right field, where Josh Harrison made a leaping catch at the wall — for the second straight night, the Pirates sealed the game with a spectacular play.

“We’re definitely going through a little bit of a tough patch right now,” center fielder Denard Span said. “We just haven’t been able to catch any breaks. Guys are diving for balls left and right the last three or four games and taking away some key hits that could change the game. But we’ve just got to weather the storm.”

It was one last missed chance. The Nationals went 1 for 9 with runners in scoring position and marooned 11 men on base. At one point, their runners-in-scoring-position streak reached 0 for 25, dating from the Nationals’ splattering of Cincinnati’s Johnny Cueto in the seventh inning Tuesday night.

“When we get in those situations, we have to just maybe take a deep breath, relax and remember the pitcher on the mound, he’s the one on the ropes,” Span said. “I’m guilty of it as well — all of us. We just have to bear down a little bit, be selective in those situations. All you can do is put a good swing on the ball and hope that it falls. And right now, it’s just not falling.”

Zimmermann led off the top of the fifth with a single, and Span smoked a line drive at Ike Davis, who stepped on first for a double play.

“Something’s got to turn,” Zimmermann said. “We’re hitting balls on the screws and right at guys. We got guys on base almost every inning. We’re just not getting the clutch hits right now. It’s going to come. It’s a long season. Can’t be too upset right now. We’re still .500.”

As the Nationals flailed early, stranding two in the first and wasting Span’s leadoff double in the third, the Pirates damaged Zimmermann. Davis rolled an infield single to lead off the fourth. With one out, Zimmermann hung a 1-0 change-up to Alvarez. He blasted it to center field, and the ball disappeared in the shrubs behind the fence. The Pirates had taken a 4-0 lead, too much for the Nationals to recover from.

“If I could take one pitch back, it would be the change-up to Alvarez, and we win the game, 3-2,” Zimmermann said. “If I get that down and away where I want it, he’s rolling over, potentially a double play. I just left it up. That’s what’s going to happen when you throw that pitch middle.”

The Nationals gave themselves so many chances to come back, and they only made them yearn more for the hitters they don’t have.

“There’s no question we want to have those guys back,” Span said. “But once 7 o’clock starts, I’m not thinking about that. We’ve got to find a way to win. That’s why there’s a 40-man roster. Guys have got to pull up some slack. Guys are getting opportunities now. Even as a young player, this is what you want. We’re a good team. You want to step in and try to help us win right now. We’ve just got to try to hold the fort until we get our guys back.”