Marc Rzepczynski was the first reliever in the game, throwing 1 1/3 innings with three walks and two strikeouts. (Katherine Frey/The Washington Post)

In the eighth inning Sunday afternoon at Nationals Park, Manager Dusty Baker took the ball from Blake Treinen and patted the right-hander on the shoulder, gratitude for his part in stifling the Los Angeles Dodgers. The pitching change offered first baseman Ryan Zimmerman the opportunity for brief reflection. Standing on the mound as he watched Treinen march into the dugout, he looked toward second baseman Daniel Murphy and nodded toward Treinen.

“He’s grown up a lot,” Zimmerman said later.

Treinen, then, represented the entire Washington Nationals bullpen. The Nationals’ prospects this fall may hinge on the evolution of their relief corps, and their 5-2 victory in Game 2 of the National League Division Series provided eye-opening promise. This may be difficult to process after the Nationals’ previous two October excursions, but their bullpen may be the thing that propels them into the next round.

Their bullpen failed them, in spectacular fashion, in their 2012 and 2014 trips to the NLDS. On Sunday afternoon, it seized control and held on tight for the entire second half, in a manner that suggested it can do so again throughout October if it has the chance.

“If you look across baseball, when you get in the playoffs, you’re going to run into bullpens that are nasty,” left fielder Jayson Werth said. “That’s how you win.”

It’s how the Nationals evened the series Sunday. After Tanner Roark’s uneven start, five Nationals relievers, two of whom they acquired around the trade deadline, combined for ­4  2/3  scoreless innings. In order, Marc Rzepczynski, Sammy Solis, Treinen, Oliver Perez and closer Mark Melancon held the Dodgers to three walks and one hit while striking out five. In the series’ first two games, they have pitched 72/3 innings without allowing a run.

Based on recent Octobers, it may take a few more games before fans lose the urge for a trip to the liquor cabinet when the bullpen gate swings open. But this is a different bunch than the 2012 or 2014 versions, overhauled not only in ability but personnel. None of the Nationals relievers who pitched in the 2014 NLDS remain on the roster.

“It was obvious: We had to shore up the bullpen,” Werth said. “We really made it a priority to get that bullpen where it needed to be.”

During their ascension to annual contention, the Nationals have built excellent bullpens for the regular season. The postseason history is etched in the darkest recesses of a fan’s mind: Drew Storen’s ninth-inning implosion cost them the 2012 NLDS against the St. Louis Cardinals, and his inability to finish Jordan Zimmermann’s gem in 2014 led to a 2-0 hole. That series ended with Aaron Barrett’s meltdown, aided by Matt Williams’s managerial misadventures.

Closer Mark Melancon, right, hugs catcher Jose Lobaton after getting the final three outs for his first postseason save with Washington. (Katherine Frey/The Washington Post)

“Our bullpens have been good,” Zimmerman said. “But experience, once you go through it a few times, experience is big in the bullpen. Regular season and the playoffs is a different thing. Walking people in the playoffs is a killer.”

Over the winter, the Nationals signed Shawn Kelley to a three-year deal and lefty specialist Perez for two years. The real work came at the trade deadline, well after it became painfully clear Jonathan Papelbon was a failed experiment at closer.

As Rizzo discussed Aroldis Chapman with the Yankees, he engaged in parallel negotiations with the Pittsburgh Pirates about Melancon. As the Yankees’ ask exceeded his appetite for Chapman, Rizzo pulled the trigger for Melancon, surrendering young lefty reliever Felipe Rivero and pitching prospect Taylor Hearn.

Melancon may have felt like a letdown compared with landing Chapman, but inside the Nationals’ clubhouse, they celebrated. Zimmerman pointed out nobody has saved more games than Melancon in the past three years. Werth had faced him two weeks earlier at Nationals Park, grounding out to shortstop.

“I remember being like, ‘God, this guy is still really nasty.’ Everything moves late. He throws strikes, and his stuff moves late. Those are the guys that are tough because everything is off the barrel. I remembered thinking when we got him, ‘This guy is a huge addition.’ I was pumped.”

The feeling only grew as the season wore on. “He’s an awesome teammate,” Werth said. He’s prepared. He’s kind of a no-frills guy, pretty quiet. Then he gets out there, he’s a killer.”

In the final week of the season, Zimmerman chatted during a game with Pirates first base coach Nick Leyva. “He said how much their bullpen and their guys miss him because of what kind of leader he was, just the rock that he is,” Zimmerman said. “He’s one of the under-the-radar pick-ups. Him and Rzepczynski.”

The reliever they call “Scrabble” gave the Nationals a third left-hander and crucial experience, a member of the 2011 World Series champion St. Louis Cardinals. With two additions, the Nationals had a bullpen with varied looks — Treinen’s high-90s sinkers, Kelley’s wipeout slider, Perez’s funk and finesse, Solis’s power from the left side, etc. — able to fill varied roles.

“It’s very similar” to the 2011 Cardinals, Rzepczynski said. “You’ve got guys that can get one hitter or at the same go get multiple innings if you need to. Any one of us down there can do that. And it all starts in the back end, having a closer like Mark. It’s similar from the back end down.”

The man choosing the relievers has improved, too. Baker, maligned for his use of the bullpen in some corners, made a series of flawless choices Sunday. He navigated 14 outs without using Kelley or precocious long reliever Reynaldo Lopez and without any relievers recording more than four outs. Even with another game Monday, the relievers will be fresh behind Gio Gonzalez, a starter prone to short outings.

The most important move may have come in the sixth, when Rzepczynski walked Adrian Gonzalez on his 34th pitch. With left-hander Josh Reddick up, Baker called for Solis, a lefty, to replace a lefty. In Game 1, Solis had pitched two full innings. In Game 2, he only faced Reddick — and retired him on a liner to left, stranding two runners in a 4-2 game.

“I thought that was a pretty slick move,” Werth said. “That was a big part of that game. The old man knows what he’s doing.”

The Nationals trust Baker to make the right moves. For the fans in the seats, it may take longer to trust an October bullpen. So far, at least, they have seen reason to give it a shot.

On Baseball: Nationals bullpen comes up big in Game 2 of NLDS