WASHINGTON — The New York Mets starting rotation remains fluid with 18 games remaining in the regular season. But it doesn’t appear as though Rafael Montero will be part of it moving forward.

He struggled with command for the third straight outing as the Washington Nationals beat the Mets 8-1 on Monday night in the opener of a three-game series.

Montero walked three Nationals in the first inning, including Bryce Harper and Wilson Ramos with the bases loaded. He lasted 1 2/3 innings for the shortest of his 12 career big league starts, allowing five hits and six runs with four walks and two strikeouts.

Manager Terry Collins was not sure who would start the next time Montero’s turn comes up, but he ruled out giving the right-hander another opportunity.

“It’s pretty frustrating, because he does have good stuff,” Collins said. “You can see when he’s actually throwing strikes, there’s not a lot of good swings. But he gets himself in trouble because he gets behind in counts.”

Over his last three starts, Montero has allowed 14 walks in 11 innings.

“It doesn’t matter where they put me,” Montero said. “I just have to continue doing my job. I think what I tried to do is just get ahead of hitters, but unfortunately sometimes things don’t work out.”

Collins did not rule out the possibility of Jacob deGrom taking Montero’s spot in the rotation, although deGrom may need more time to recover from forearm soreness that has kept him out since Sept.1. He threw a 35-pitch bullpen session at Nationals Park Monday afternoon.

“Anybody could be in the mix for that spot,” Collins said.

Mat Latos threw 4 1/3 innings in his first start with the Nationals allowing three hits and one run with three walks and four strikeouts. His first start with the Nationals was cut short due to hamstring tightness. Reynanldo Lopez (3-3) combined with three relievers for 4 2/3 scoreless innings.

Montero struggled early Monday walking three Nationals in the first inning, including Bryce Harper and Wilson Ramos with the bases loaded.

Nationals pitcher Mat Latos led off the second with a solo homer over the left field fence, before Daniel Murphy doubled and Harper was intentionally walked. The move backfired as Rendon then delivered his 18th homer of the season to put the Nationals ahead 6-1. It was the 60th and final pitch of Montero’s outing.

Jose Reyes hit a leadoff double in the first and later scored on a Yoenis Cespedes RBI sac fly to give New York a short-lived 1-0 lead.

Ramos and Ryan Zimmerman added RBI singles for Washington off Mets reliever Gabriel Ynoa.

With the win, the Nationals increased their lead in the National League East to 10 games – their largest cushion of the season- and reduced their magic number to nine to clinch their third division title in five years.

BACKMAN NOT RETURNING:

Former Mets infielder Wally Backman will not return as the organization’s Triple-A manager in 2017. Mets general manager Sandy Alderson told reporters that Backman “has decided to move on,” after working in New York’s minor league system since 2010. Backman spent the last four seasons as manager of the Las Vegas 51s.

TRAINER’S ROOM

Mets: Infielder Wilmer Flores missed his second straight game with a stiff neck. Manager Terry Collins said that Flores tried to hit Monday afternoon but his neck remained sore. Flores was injured Saturday after a home plate collision with Atlanta Braves catcher A.J. Pierzynski.

Nationals: Manager Dusty Baker is optimistic that RHP Joe Ross (shoulder inflammation) will soon be reinstated from the 60-day disabled list. Ross, who last pitched for the Nationals on July 2, threw a simulated game Saturday after making two rehab starts with Triple-A Syracuse.

UP NEXT

Mets: RHP Noah Syndergaard (13-8) looks to snap a personal three-game losing streak against the Nationals as he makes his fifth start of the season against the NL East leaders. Syndergaard threw seven innings and allowed two runs on three hits in a 4-1 loss to Washington Sept.2 at Citi Field.

Nationals: RHP A.J. Cole (1-1) will make his fifth start of the season Tuesday and his second against the Mets. Cole’s best outing came Sept.2 in New York when he allowed one run and three hits over six innings for his first career Major League win.