Rookie Joe Ross turned in another quality start, striking out seven in the Nationals’ 8-3 win Thursday over the Diamondbacks. (Brittany Greeson/The Washington Post)

Hours before the Washington Nationals took the field Thursday afternoon, with questions swirling about their uneven play in the final third of the season, Manager Matt Williams gave the team a message. Since he took over as manager, he has preached aggressiveness on the base paths, in the batters’ box and on the mound. And recently, the Nationals have not been that — and at the same time they have lost ground in the division race by playing to avoid losing, not playing to win.

“When we’re an aggressive team, we’re at our best,” Williams said.

Thursday’s 8-3 win over the Arizona Diamondbacks was an improvement. A sluggish offense slowly came to life thanks to contributions throughout the lineup: power from Ian Desmond and Clint Robinson, key hits from Wilson Ramos and Ryan Zimmerman, Bryce Harper’s eye and, of course, heads-up base running.

But the Nationals’ most aggressive move was announcing that rookie Joe Ross, who tossed yet another strong outing, would remain in the rotation when Stephen Strasburg returns from the disabled list Saturday. The move pushes struggling veteran Doug Fister to the bullpen.

“It’s never easy, but he’s willing to go out there and do what he can to win,” Williams said.

While fan favorite Drew Storen has struggled closing playoff games, Jonathan Papelbon has proven himself to be more consistent. Who's more suited to pitch the ninth inning of a playoff game? (Post Sports Live/The Washington Post)

Before the Nationals moved within 1 1/2 games of the idle New York Mets in the National League East standings, the Diamondbacks posed a challenge to Ross. They sat two all-stars, Paul Goldschmidt and A.J. Pollock, and power-hitting catcher Welington Castillo and stuffed the lineup with five left-handed hitters. Entering Thursday’s game, left-handed hitters hit .300 against Ross, and his neutralizing change-up was still a work in progress. But Ross again rose to the occasion.

“He hit the outside corner,” Williams said. “He didn’t give them much to pull. Used a back-foot breaking ball a couple of times for strikeouts. He was in command.”

Ross was a wrecking ball to right-handers — he has yet to allow an extra-base hit to them in the majors — and gave up four hits in the 14 at-bats against left-handers. Only one, a solo home run by Jarrod Saltalamacchia in the fifth inning, did any damage.

“He’s been better than anyone really expected,” Zimmerman said.

In backing Ross, the Nationals’ lineup was ignited in the second inning, revved up in the fifth and took off in the eighth. After a day off to rest his body and mind from offensive struggles, Desmond crushed a first-pitch change-up from Jeremy Hellickson for a solo home run, fitting with Williams’s message.

“It’s short, compact and he was calm at the plate, which is important for him, and put the head [of the bat] on the ball,” Williams said.

The Nationals took a 2-0 lead in the fourth inning after Desmond scorched a double, Ramos singled and Michael A. Taylor brought home a run with a groundout. After Ross allowed a run, the Nationals instantly responded by taking a larger lead.

Harper, who reached base five times, singled and advanced twice — once on a wild pitch and once on a passed ball. Jayson Werth reached on the wild pitch despite striking out. Desmond walked to load the bases. With two outs, Ramos drilled the first fastball he saw into left field to give the Nationals a 4-1 lead.

The Nationals nearly lost their grip in the seventh inning. Casey Janssen surrendered two runs, his first in nine outings, and the Diamondbacks pulled within a run. But Matt Thornton, inheriting runners on the corners with only one out, got a flyout and groundout to wriggle out of the jam.

“Oh [shoot],” Thornton said he thought to himself when he entered in a tough spot.

Taylor’s aggressiveness helped the Nationals add to their lead. He was hit by a pitch and then stole second base, both plays that went to a review but were upheld. Zimmerman then gave the Nationals a two-run lead with a pinch-hit double. An inning later, Robinson smashed a no-doubt three-run home run.

“Hitting can be contagious,” Robinson said. “So taking advantage and when you get something, do something with it.”

That allowed Williams to turn the ball over to Drew Storen, whose Nationals record of 19 straight batters retired in relief was snapped with a walk in the eighth, and recently returned Blake Treinen. Williams’s message of aggressive resonated with some in the clubhouse.

“That’s very important because we need confidence,” Ramos said. “When the manager comes to us and tries to give us confidence, we feel better. We feel better to go out there and play hard. You have to be together. Manager, players, staff. We need to be together. This is a team. This is a family. And if we can support each other, we can be better.”

After the game, the Nationals made public their decision for the rotation. After making two rehab starts, Strasburg will return from an oblique strain that has held him out since July. Fister’s 4.60 ERA is the highest among the healthy Nationals starters, and he still hasn’t quite pitched like himself since returning from a forearm injury June 18. Ross may not have the experience of Fister, but he has a 2.80 ERA with 47 strikeouts and four walks in 45 innings spread over seven major league starts, along with youth and poise on his side.

“The five great starters that we have and Strasburg coming back so for me to stay, I’m gladly going to take the role and continue to help the team,” Ross said.

Longer term, a complicating factor is Ross’s inning limit. Ross, who turned 22 in May, has not thrown more than 122 innings in a professional season. Williams said the Nationals haven’t determined when Ross will be shut down, and it is unclear whether Fister could still start after that happens. Including six innings Thursday, Ross is up to 121 innings between minor and majors this season.

“When they pull me, that’s it,” Ross said. “I’ll just keep going for now.”