Roark will try to get the NL East leaders untracked by winning a fifth straight start Friday night against the Chicago Cubs.
Ramos has lost 53 games to injuries this season, while Harper has been on the disabled list since April 27 due to left thumb surgery. The star left fielder could be back Monday against Colorado, and Washington (41-37) will be hoping for a smooth transition.
They got that from Ramos on Thursday when he returned from a 14-game absence due to a hamstring injury and had two hits while scoring a run in a 5-3 loss to the Cubs (33-44).
While Ramos has two hits in each of his past three games and a .364 average over his last 14, the Nationals' chances of bouncing back in this four-game set could rest on Roark (7-4, 2.79 ERA).
The right-hander leads the team in wins and has already matched his total from last season. He's won four straight starts behind a 1.48 ERA and with the help of 17 total runs of support. He got 11 over his previous five outings - a big reason why he went 1-3 despite a 2.20 ERA.
Roark's recent success continued Sunday during a 4-1 win over Atlanta in which he yielded one run and four hits while battling through 96 pitches in 5 1-3 innings.
"He wasn't throwing it exactly where he wanted to, but he was able to work through it," manager Matt Williams said. "He wanted to stay in the game when I went and got him."
Roark's only meeting with the Cubs came out of the bullpen at Wrigley Field on Aug. 21, when he was credited with an 11-6 win despite giving up two runs in 1 2-3 innings.
Denard Span is 6 for 17 (.353) with four RBIs over his last five games there. The center fielder doubled twice and drove in two runs Tuesday, but was involved in a bizarre incident when he lost a fly ball in the fog.
"Just rough conditions," Span said. "Nothing you can do to prepare for that. I don't think I've ever played in a game with that much fog."
Despite winning the series opener, Chicago has dropped four of six while hitting .198.
Anthony Rizzo is providing a rare bright spot, going 14 for 32 (.438) with three homers and six RBIs in the last eight games. He had three hits Thursday and owns a .375 career average versus Washington.
Jason Hammel (6-5, 2.99) will also look to continue his success against the Nationals. The right-hander is 6-0 with a 3.15 ERA in eight career starts, winning the last two with Baltimore while allowing three runs - two earned - with 18 strikeouts in 16 innings.
Hammel, who has forced Span to go 1 for 10, enters this meeting among the NL leaders with a career-best 1.02 WHIP. He was just as effective Sunday, allowing two runs and six hits in seven innings while not getting any run support in a 2-1 loss to Pittsburgh.
Despite the defeat, it was an improvement for Hammel, who had allowed eight runs and 19 hits in 11 innings while going 0-1 over his previous two outings.
"It's coming back," he told the team's website of his slider. "The command of the slider and the heater are coming back, so I feel pretty good about it."