The Nationals made official two significant decisions on Tuesday that showed urgency about their desire to catch the Mets in the National League East. First, in the corresponding move to clear a roster spot for returning center fielder Denard Span, the Nationals sent right-hander Tanner Roark to Class A Potomac so that he can be stretched out as a starter and potentially replace rookie Joe Ross as he nears his innings limit.

Secondly, and perhaps more significantly, the Nationals adjusted their rotation following Monday’s off-day so that their big three pitchers — Max Scherzer, Jordan Zimmerman and Stephen Strasburg — can face the Mets on Sept. 7-9 in Washington.

After the all-star break, the Nationals opted to give Scherzer extra rest because of a big first-half workload, but that meant he missed the Mets series in New York earlier this month. The Nationals were swept in that series after games started by Gio Gonzalez, Ross and Zimmermann. The Nationals trail the Mets by 5 1/2 games entering Tuesday’s games and know their six head-to-head match-ups will be critical. So they made official a decision they had mulled for weeks.

Scherzer was originally on track to start Wednesday against the Padres, but now Gonzalez will start that day on regular rest, followed by Ross on Thursday, missing a match-up against older brother Tyson by a day. Scherzer, on two extra days of rest, will  start Friday and Zimmermann, on an extra day of rest, will go Saturday.

“We have an opportunity when we face [the Mets],” Nationals Manager Matt Williams said. “There’s a lot of games in between that we have to win to have that opportunity. Lining it up, we’ve been talking about it for weeks now. Given where we’re at and what’s in front of us and what we’re trying to accomplish, it gives us the best option.”

The Nationals have two more off-days before the season-ending series against the Mets so more tinkering with the rotation could be in store.

Sending Roark to the minors was the toughest move, Williams said. When the Nationals signed Scherzer in the offseason, they pushed Roark, who won 15 games and had a 2.85 ERA last season, to the bullpen. He did so without complaint. Twice already this season, Roark has gone back and forth from relieving to starting when need arose. Now, he was sent to Potomac — for the minimum 10 days in the minor leagues before he can be recalled — so that he can get back into a starter’s schedule.

“It’s still not easy,” Williams said. “The guy won 15 games last year. It’s not easy to do that at any time. Often times those decisions have to be made and we have to prepare. We have to make that if in fact Joe is at the end at his limit sometime in September then somebody is prepared to do it.”

Ross has logged 136 2/3 innings between the minors and majors, eclipsing his previous high of 122 1/3 innings two years ago. (He threw 121 2/3 last season.) The Nationals have a loose innings cap on 22-year-old Ross because of his age and previous workloads.

“All indications from last start is that he’s pretty good,” Williams said of Ross. “Again, that’s a decision we’ll have to make if and when we have to make it. He’s in uncharted territory. It’s a start to start, day to day, if you will, and see how he goes from here. We want to be prepared if in fact that times comes where it’s time to shut him down where we’ve got somebody that’s stretched out and ready to go.”

The Nationals believe Roark is the best option to fill a void potentially left by Ross as opposed to using Doug Fister, who was pushed to the bullpen when he struggled. The back and forth between the rotation and bullpen has affected Roark’s routine and mentality, but he has been better as a reliever. He has a 5.61 ERA in six starts, a small sample size, compared to a 3.74 ERA in 28 relief appearances.

“He’s pretty resilient,” Williams said. “He’s been to 50-ish pitches in the bullpen and 100 as a starter. Question is making sure he’s prepared to do it. If in fact there’s need for that, he’s prepared and has been out there for 5-6 innings and he can handle it. It’s been a long time since he started so we want to make sure that pitch count is up there and he can fully get through a five-day rotation a couple times and go from there.”

Williams said Roark will not be in the minors for long and Potomac is close to Washington. Roark can toss 60-some pitches in his first start there and then build up to 80-plus. He could then rejoin the Nationals after 10 days, and by then rosters will have expanded.

“We’re not necessarily concerned about the competition that he’s going to face necessarily,” Williams said. “It’s about increasing his pitch count. So he has opportunity to do that close, where we have eyes on him and go from there. Get a couple starts hopefully and see where he is at.”