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WASHINGTON — It took almost a month into the season, but manager Carlos Mendoza finally got to fill out a batting order Friday that included everybody from the projected Opening Day lineup.

The return of Francisco Alvarez and Jeff McNeil from the injured list gave the Mets a new look as they opened a four-game series against the Nationals, albeit unsuccessfully with a 5-4 loss.

“Coming into this year, [Alvarez and McNeil] were going to be a big part of this team and this lineup,” Mendoza said at Nationals Park. “Here they are, healthy and ready to go.”

Alvarez, who went 1-for-4 with a strikeout Friday, was rehabbing from surgery to remove a broken hamate bone in his left hand. He will return to primary catching duties, but Mendoza expects to find opportunities for Luis Torrens, who shined as the starter in Alvarez’s absence.

McNeil, who also went 1-for-4, will play second base and outfield (possibly with some of that action in center), displacing rookie Luisangel Acuña from regular playing time. Acuña has been a bright spot for the Mets offensively, defensively and on the bases.

Mendoza was asked if he was concerned about disrupting the chemistry — the Mets are a major league-best 18-8 — by inserting two new names into the lineup.

“They are a big part of this team, even when they weren’t playing they were around,” Mendoza said. “And last year they were a big part of this team. That’s what it’s going to take — it’s going to take more than 26 guys and here we are getting two of the bigger pieces [back]. That’s a good group in that clubhouse and they will fit right in.”

McNeil, who strained an oblique in spring training, was set to play his first regular season game since Sept. 6 of last season.

He missed most of the final month last year rehabbing from a broken wrist after he was hit by a pitch. McNeil returned for the NLCS against the Dodgers.

“It feels like Opening Day for me,” McNeil said. “We’re playing well, and I am just going to do what I can to help these guys win.”

That potentially includes playing center field — a position at which he was utilized during his minor league rehab assignment. Tyrone Taylor has been playing center every day for the Mets with Jose Siri sidelined by a broken left tibia, but Mendoza is looking to get others involved.

“I enjoyed it,” McNeil said, when asked about playing center on his rehab assignment. “I take a lot of pride in playing a lot of positions and being very versatile. Wherever the team needs me.”

Alvarez was working on a new swing in spring training, but the Mets received only a small sample size of it in exhibition games before his injury.

“When I was doing my rehab I was working a lot on my swing to try to feel right,” Alvarez said through an interpreter. “When I wasn’t swinging I was working a lot on my mentality, on what I wanted to do. And right now I feel really good.”

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Mendoza said his goal is to also keep Torrens as involved as possible.

“They are both going to play,” Mendoza said. “[Torrens] played real well for us and he earned that opportunity. We are getting Alvy back and he’s going to get an opportunity as well. I think that’s a good problem to have, but I could see both of them playing.”

Mendoza also anticipates finding spots for Acuña to play.

“We’ll find ways to keep Acuña sharp and get him some at-bats,” Mendoza said.