WASHINGTON -- Two key members of the Nationals returned from the injured list when rosters expanded on Sunday, but both reliever Sean Doolittle and first baseman Ryan Zimmerman will be welcomed back with slightly different roles from the start of the season.

Both showed how they can still contribute to this Nationals team during a 9-3 victory over the Marlins at Nationals Park. Zimmerman made the most of his start in Sunday’s series finale by launching a two-run homer in the sixth inning, his fourth of the season and first since April 21. And Doolittle closed out the victory with a scoreless ninth, even though it was not a save situation.

“Anyone in this room, we're all competitors,” Zimmerman said. “The hardest part is when you can't be out there. I think that's the most frustrating part for any athlete in any professional sport. All of us want to be out there. Yeah, it's been frustrating. But the way they've been playing, the winning has made it a little bit easier, but you always want to be out there and be part of it.”

Zimmerman and Doolittle were two of seven players the Nationals added to their roster on Sunday, joining right-handers Austin Voth and Jeremy Hellickson, catcher Raudy Read, infielder Adrián Sanchez and outfielder Andrew Stevenson.

This marked Zimmerman’s first game since July 21 after missing the last 36 with plantar fasciitis in his right foot, his second bout with the injury this year. He received a loud ovation from the 29,345 fans in attendance during pregame introductions and before his first at-bat, a show of appreciation for Zimmerman as he tries to contribute to this postseason push. Zimmerman is expected to split duties at first base this month: starting against lefties and coming off the bench as a pinch-hitter or defensive replacement.

Zimmerman played in eight games during his latest Minor League rehab stint, going 10-for-25 (.400) with three doubles and five walks. His bat also showed promise in the nine games before his latest stint on the IL, when he hit .333 with five doubles and hit safely in eight of the nine games.

On Sunday, Zimmerman's two-run homer seemed almost cathartic, a 437-foot dinger swatted at 110.6 mph off the bat, complete with a dugout dance that included the veteran Zimmerman pretending to use a walking stick.

Nats fans will have to get used to seeing Doolittle, who spent two weeks on the IL with right knee tendinitis, pitching in situations like Sunday's considering he will not be the team's closer right away. Prior to Sunday's game, Doolittle said he felt good enough to help out of the bullpen in some capacity, but he does not believe his stuff is sharp enough to return to high-leverage situations immediately.

Doolittle and Martinez spoke at length this weekend about his role upon returning, trying to gauge his feelings after a simulated game and Minor League rehab appearance this week. Even though Doolittle came away from them feeling good, he believes working in lower-leverage scenarios will help him ultimately get back to form.

“For now, stuff-wise, I'm not ready to be at the back end of a game,” Doolittle said. “But I still think I can help the team. Keep guys like [Daniel Hudson] and [Hunter Strickland] and [Fernando Rodney] -- I can keep those guys fresh. I can help get them the ball. I can match up against lefties, with an eye on hopefully getting back into one of those roles in the back end of a game at some point here in the next couple weeks.”

Doolittle had been one of the best closers in the Majors last season and through the start of this year before a rough patch put him on the IL to heal an injured knee and slow down his pace for a career-high number of appearances. Martinez plans to ease him back into games for now, but he made it clear he expects Doolittle to end up taking over as the closer at some point in the future.

“I told him, I said, 'In order for us to be super effective, we need you to be that guy again,'” Nationals manager Dave Martinez said prior to the game. “We'll get there. 'Take baby steps. You're healthy, just build you back up and once we feel you're there -- that'll come.' I need him to believe that that will happen.”

Something to prove: 1B Ryan Zimmerman, LHP Sean Doolittle, RHP Jeremy Hellickson

This category is where the majority of the team’s September callups will fall, players who have not had the season they planned on at the start of the year and could use the month to erase it.

Zimmerman has been limited to just 33 games during what has been one of the most frustrating seasons of his career. He returns as part of a platoon at first base with left-hander Matt Adams, but if he can get hot, the Nats could lean on him once again.

Doolittle’s 2019 has not been a complete disaster, but he was in the midst of one of the worst stretches of his career before landing on the IL. In eight appearances in August, Doolittle allowed five home runs and posted a 12.86 ERA.

Hellickson began the year as the team’s fifth starter, but he appeared in just nine games (eight starts) with a 6.23 ERA before landing on the IL on May 21. He is expected to be available out of the bullpen as a long reliever this month and could receive a spot start.

Arrival: INF Carter Kieboom

After a brief appearance in the Majors this year, it’s unclear whether the Nationals plan to call up top prospect Carter Kieboom for a second taste of the big leagues in 2019, but his numbers at Triple-A Fresno prove he is ready for the call. He batted .303/.411/.496 with a .907 OPS and 16 home runs in 107 games. Although he is unlikely to crack the Nats’ postseason roster, he could be promoted to get more Major League experience and get a start or two to give the Nationals’ infielders a breather.

Breakout: RHP Austin Voth

Voth entered the season as low as eighth or ninth on the team’s starting-pitching depth chart, but at one point this year, he had the fifth spot in the Nats’ rotation before landing on the IL. Voth made four starts and posted a respectable 108 ERA+ with 20 strikeouts and six walks in 20 2/3 innings.

Name to watch: INF Luis Garcia

Garcia won’t turn 20 years old until May, but he already received an invite to his first big league camp and will play in the Arizona Fall League next month. Washington's No. 2 prospect had a bit of a down year offensively this season, batting .259/.282/.340 at Double-A Harrisburg, but he is still way ahead of the curve for a player his age. With a strong start to next season, Garcia could make his debut and an impact on the 2020 Nationals.