Marc Rzepczynski is on a contender again. (Tony Gutierrez/Associated Press)

Marc Rzepczynski boarded a flight from San Francisco to Dulles on Thursday still a bit shocked. He had been traded for the fifth time in his professional baseball career, but this was the first after the non-waiver trade deadline, when players must pass through waivers before they can be traded. Suddenly, he had jumped from a bad Athletics team to the Nationals, the club with the second-best record in baseball with five weeks remaining in the regular season.

“This one got me a little bit,” Rzepczynski said. “I was actually taking a nap when this happened. We had a day game, so I was just relaxing. It was kind of a shock, but I’m definitely happy to be here.”

Nationals General Manager Mike Rizzo wanted him to report to Nationals Park in time to bolster a taxed bullpen Thursday night. An hour flight delay jeopardized the plan, but the Nationals were confident that he would arrive on time and announced that Rzepczynski was added to the 25-man roster before the game in place of Reynaldo Lopez.

Rzepczynski made it by 8:30 p.m. It was the fifth inning of the Nationals’ 4-0 win over the Orioles. In the eighth inning, he was warming up alongside Shawn Kelley in case Max Scherzer stumbled. Scherzer didn’t, so Rzepczynski didn’t have to enter the game to pitch to a catcher, Jose Lobaton, he had not even met yet.

Rzepczynski, 30, is in Washington as a two-month rental (he’s a free agent this winter) because the Nationals needed a quality left-hander in the bullpen. After Felipe Rivero was traded to Pittsburgh, Oliver Perez has struggled through a neck stiffness, and Sammy Solis is on the disabled list.

In 56 relief appearances with the Athletics, Rzepczynski posted a 3.00 ERA. He has been better against right-handed hitters than previously this season but worse against left-handed hitters, which is unusual for a pitcher who has made a living as a left-handed specialist. But luck wasn’t on Rzepczynski’s side early in the season — left-handed hitters posted a BABIP over .400 against him — and his numbers have improved recently.

“I think it was a combination of just going after and attacking guys,” Rzepczynski said.“I think it was also a combination of luck, too. I felt like I was making some pretty good pitches earlier in the year. My walks were a little up, and I’ll try to contain that as much as I can. Just trying to go after guys strike one, trust my stuff and let the defense play behind you. A little bit earlier in the year, they were finding some holes, so the numbers were a little bit skewed. But at the same time, I was getting righties out. It was one of those years where early on I was a little confused, but I feel like I’m back on track now.”

Rzepczynski’s gameplan is not a secret: He hammers hitters with a sinker. They know it’s coming. Often, it doesn’t matter. The southpaw’s ground-ball rate, 69.5 percent, is the second-highest in baseball among pitchers that have logged at least 30 innings. Only Orioles closer Zach Britton, who on Wednesday allowed his first earned run since April, boasts a higher percentage.

It was earlier in the season and not a complete shock, but Rzepczynski was traded from a middling club to a playoff contender before. It was in 2011, and he went from the Blue Jays to the Cardinals. The Cardinals won the World Series in seven games, with Rzepczynski appearing in four of them and making 12 overall appearances in that postseason. He has a chance to replicate the run five years later.

“It’s amazing. At the end of the day, with a team that’s losing, you’re making plans for the offseason already. Then, all of a sudden, I’m playing in October, or hopefully I have a chance to play in October,” Rzepczynski said. “It was one of those where it reminded me a lot of ’11 when I was with the Cardinals. Being traded at the deadline, then all of a sudden winning the World Series. So, hopefully I’ll have the same thing happen here.”