Nationals starter Blake Treinen drills Pittsburgh’s Andrew McCutchen square in the back with a third-inning curveball to drive in the game’s first run. (Justin K. Aller/Getty Images)

Flat on his back, pressed against the outfield grass, Andrew McCutchen held the ball up in his glove, like one more light atop the Roberto Clemente Bridge. The PNC Park crowed roared the game-ending out, the last act of the reigning MVP’s assault on the Washington Nationals, the final attempt of an offense enfeebled by a mixture of stellar defense, rotten luck and poor execution.

McCutchen’s career-long barrage against the Nationals continued apace in Washington’s 3-1 loss to the Pittsburgh Pirates on Thursday night. He drove in the two runs that mattered most and ending the game by snaring Anthony Rendon’s liner to center with a sliding catch, skimming the ball off the top of the turf.

“He’s the MVP for a lot of reasons,” Nationals Manager Matt Williams said. “He’s a good player, a really good player. He’s not going to go after that ball unless he feels like he’s got a chance to catch it.”

McCutchen entered with a 1.283 OPS and 13 home runs in 30 games against the Nationals. They retired him only once Thursday, when he smashed a line drive into the pitcher’s glove in the first. But McCutchen’s production alone did not have to lead to a loss.

The Nationals’ last failed rally finalized the squandering of the second major league start from Blake Treinen, the right-hander with a high-90s sinker who has temporarily replaced Gio Gonzalez in the Nationals’ rotation. Treinen allowed two runs in 52 / 3 innings, surviving a brief implosion in the third inning and falling behind for good when McCutchen ripped an RBI single up the middle in the fifth.

Ross Detwiler has struggled in the bullpen this season after losing his spot in the Nationals’ starting rotation. The Post Sports Live crew offers advice on what to do with the hard-throwing left-hander. (Post Sports Live/The Washington Post)

“He handled me pretty good tonight,” Treinen said. “I just need to execute pitches a little better next time. My stuff plays. I just have to execute pitches. Next time I’m out, I’ll do a better job of it.”

Treinen gave the Nationals everything they could have hoped for from a rookie fill-in. They gave him almost nothing in return.

For the second straight day, the Nationals scored once in a game started by a right-hander. The Nationals went 0 for 8 with runners in scoring position, managed five hits and left the bases loaded in the eighth inning. Their lone run came when Ian Desmond blasted a home run in the fourth. First baseman Adam LaRoche started a rehab assignment at Class A Potomac, and the Nationals cannot get his bat back in the lineup fast enough.

Against starter Edinson Volquez, who entered with a 4.71 ERA, the Nationals mustered one run and just three hits over six innings. They put runners on first and second with no outs in the third, but three consecutive outs, including Jayson Werth’s liner to McCutchen on the warning track, spoiled the rally. Otherwise, they didn’t even threaten.

“[Volquez] had decent stuff,” Desmond said. “I’m not going to give him the Cy Young or anything.”

The Nationals, at times, fell victim to the Pirates’ ever-shifting and athletic defense. McCutchen’s catch served as a highlight. After Danny Espinosa and Denard Span drew walks against closer Mark Melancon, Rendon blistered a line drive to center, not dissimilar to the hit Billy Hamilton robbed from him Monday night at Nationals Park. McCutchen read it perfectly and darted toward the infield. He slid and grabbed the ball barely an inch from the grass. Earlier, right fielder Josh Harrison had robbed Wilson Ramos of a double with an all-out, horizontal dive.

“We’re not playing alone,” Ramos said.

“They made a couple really good plays on us,” Desmond said. “We hit some balls good that were right at them. But that’s part of it. Sometimes you get those bloopers and score a couple runs. But I feel like we were putting up good at-bats throughout the course of the game. It’s just not producing right now, but it will come.”

Nate McLouth could have sparked a rally when he bunted for a single in the seventh. Williams has spoken often about the importance of “manufacturing” runs with three key hitters on the disabled list. With one out and Kevin Frandsen up, here was a chance to try — McLouth stole 30 bases last season, and right-handed reliever Jeanmar Gomez had allowed four steals this year.

“He had the green light,” Williams said. But McLouth stayed put, and Frandsen rolled into a 6-4-3 double play.

The Nationals, who fell to 24-23 and into third place in the NL East (21 / 2 games behind first-place Atlanta), mounted a real threat in the eighth against left-handed reliever Tony Watson. With two outs, Werth walked, Ramos singled through the hole on the right side and Desmond loaded the bases with another walk. Williams sent pinch hitter Scott Hairston to the plate.

His presence represented a hopeful development for the Nationals — Hairston walked to the plate 7 for 13 with two doubles against left-handers. Watson had trouble finding the strike zone, but Hairston bailed him out. He popped up a shoulder-high fastball to shallow left, and Jordy Mercer squeezed it to end the inning.

“At times, I get on top of that ball,” Hairston said. “I just missed it. I saw it good. I thought I put a good swing on it. Maybe another quarter inch on top, that ball leaves the yard.”

Treinen’s effort had gone to waste. He steamrolled through his first two innings, featuring his high-90s, heavy sinker. With two outs in the third, he had retired all eight batters he faced. There had been hints of troubles — three batters had hit line drives at leather. But nothing suggested a coming meltdown.

Treinen walked Volquez, and Harrison smoked a single into left field. Neil Walker worked a walk. Suddenly, the bases were loaded, McCatty was trudging to the mound and McCutchen was striding to the plate.

“I just lost feel for a little bit,” Treinen said. “I just had to reset myself and try to find the zone again. Just let the game speed up a little bit. I just need to slow it down next time.”

Unless McCatty’s message to Treinen was, “hit him with a curveball,” then he failed in his execution. His 81-mph curveball splatted against McCutchen’s back, and the game’s first run ambled home.

“It’s frustrating to hit somebody with bases loaded,” Treinen said. “That really bothers me.”

Treinen settled and controlled the game until McCutchen came up again and whacked an RBI single through the middle. The Nationals had fallen behind, 2-1, the kind of deficit that should be vulnerable. On Thursday, for an offense without its main parts, it was plenty.