The Nationals were 4-5 during the trip and clinched the National League East title on Saturday night with a 6-1 win in Pittsburgh.
Washington (91-64) can now turn its attention to holding down the second-best record in the National League. That would assure the Nationals of hosting Games 1 and 2 of the National League Division Series on Oct. 7 and 8 at Nationals Park.
It appears likely the Nationals will face the Los Angeles Dodgers in the playoffs.
"That is important. We have to finish up strong. These games are all meaningful and we have to approach it like that," left fielder Jayson Werth told Mid-Atlantic Sports Network after Sunday's 10-7 win over the Pittsburgh Pirates.
The starting pitchers Monday will be Washington right-hander Tanner Roark (15-9, 2.70) against Arizona right-hander Archie Bradley (7-9, 5.02). Roark is going for a career-high 16th win after he won 15 games in his first year as a starter in 2014.
"We will be back to work tomorrow," said Werth, who tied the game Sunday with a homer in the eighth.
Roark was the hard-luck loser on Tuesday when he gave up one run and three hits in seven innings in a 1-0 loss at Miami.
Bradley allowed eight runs (seven earned) in 3 1/3 innings in a loss to Washington on Aug. 1. But he has given up three runs or fewer in five of his last six starts and is 3-1 with a 4.94 ERA during that run.
The Nationals may get a break in the series as Arizona right-hander Zack Greinke (shoulder soreness) may not make a start anymore this year.
"If he doesn't start Tuesday, there's probably a good chance he doesn't start again," Arizona manager Chip Hale said Sunday. "That's my opinion right now, but we'll have to get some input from the doctors."
Greinke is 13-7 with a 4.37 ERA in 26 starts this year.
"I know we were happy with the MRI results. It's just more swelling in there. You can't really go full speed until the swelling goes down, so it's hard to say he's going to be able to start," Hale added.
Washington will try to get healthy in the last week of the season.
Daniel Murphy, an MVP candidate, has missed several games recently with a strained buttocks.
Right fielder Bryce Harper hurt his left thumb Sunday in Pittsburgh when he slid into third in the third inning with a triple. Harper stayed in the game and scored a run but was replaced in right by Chris Heisey in the bottom of the third.
"His hand is sore. We are going to take care of it. It is day-to-day," Washington manager Dusty Baker told reporters in Pittsburgh. Baker, who was not pleased with the fake tag, said Harper hurt his thumb and may get an X-ray Monday.
Hale, meanwhile, feels like his team is battling against playoff-bound foes.
"They were fighting their hearts out like some of the teams we've been playing and our guys matched in," Hale said after a 2-1 loss Sunday in Baltimore.
The series with the last-place Diamondbacks (64-91) will feature the return of Matt Williams, who was the manager of the Nationals for the 2014 and 2015 seasons. Williams was on the coaching staff of the Diamondbacks before he became the Washington manager and he is back with Arizona as a third base coach under Hale.