PHILADELPHIA — A day before the Mets’ 6-2 victory over the Phillies on Sunday, Neil Walker shouted across the clubhouse, making his opinion known about a proposed rule change to the team’s long-standing fantasy football league.

Walker knew he might be traded. Still, he did not act like a man on the move, a display of practiced cool that comes only with time. But one day later, he was suiting up for the Brewers and the Mets plugged along without yet another veteran who had helped set the agenda in the clubhouse.

Such is the reality of the phase the Mets have entered in the final weeks of this season.

General manager Sandy Alderson said he has handled the dismantling of the roster as “dispassionately as possible,” taking his lead from players, who have been trained to remember that the game is in fact a business.

The start of the Subway Series on Monday night might provide a little extra intrigue in a season long lost to injuries. But manager Terry Collins didn’t bother to manufacture over-the-top enthusiasm for the annual meeting of crosstown rivals.

Said Collins: “They’re games we’ve got to go play.”

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A roster that had been crafted to contend for the pennant has been reduced to a mishmash of players on the opposite ends of the spectrum, hardly what the Mets envisioned out of spring training.

On one side are Dominic Smith and Amed Rosario, prospects getting their first taste of the big leagues. On the other are Asdrubal Ca brera and Curtis Granderson. Like Walker, both have established themselves as leaders in a clubhouse that once teemed with experience.

“It’s very important when you’re playing for postseason that you have guys who know what it’s about,” Collins said. “Neil Walker had that experience in Pittsburgh. He was a winning player, he was a leader on the field, off the field. Loved to play, loved to play hard. He was a real pleasure to have here.”

But the veteran core that Walker had helped to anchor soon could be whittled away even more. Granderson and Cabrera also are in the final weeks of their deals, meaning they are prime candidates to be moved, joining Jay Bruce, Lucas Duda and Addison Reed before them.

Granderson bolstered his stock Sunday, going 3-for-5 with a two-run homer. He knocked in three runs and scored three, continuing what has been a strong season after an atrocious April.

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Of course, as the Mets (53-62) play out the remainder of their inconsequential season, the focus has shifted more toward developing the young talent that will be expected to help with the turnaround.

Michael Conforto hammered a two-run shot, his third homer in the four-game set. His surge coincided with his shift from leadoff to the middle of the order, an especially meaningful move given the purge of veterans. Conforto has 26 homers, 12 since the All-Star break, in a season that began with him relegated to the bench after a brutal 2016.

“It’s exciting. I think that’ll be kind of a cherry on top along with putting runs on the board and helping the team win,” Conforto said about closing in on what would be his first 30-homer season. “I guess we’ll talk about it when we get there.”

On-the-job training continued for Chris Flexen (2-1), who allowed two runs in five innings. Flexen’s fourth start since being promoted from Double-A Binghamton included four walks.

A baserunning blunder by Odubel Herrera helped Flexen escape a bases-loaded, none-out jam in the fifth. Flexen had a career-high five strikeouts, an especially promising sign for the 24-year-old, who has had trouble putting away hitters on two-strike counts.

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Paul Sewald and Chase Bradford, both of whom began the year in the minors, each tossed a scoreless inning.

With that, the Mets took three of four ahead of a matchup against the Yankees, which surely will take on a different feel. Trading Walker left the Mets’ roster with an average age of 27.8, the sixth youngest in all of baseball.

Experience no longer is a given. And for Rosario, Smith, Bradford and Sewald, this Subway Series will be their first.

“It’s going to be exciting,” Rosario said through a translator. “There’s going to be a lot of fans and obviously a little bit more pressure.”

The results won’t have much impact on the standings for the Mets. But Collins called the introduction an “important” step in the young players’ development.

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“When they walk into Yankee Stadium tomorrow, it’s going to be a new experience for them,” he said. “If they’re going to play and be championship-caliber, they’re going to have to learn to deal with that kind of atmosphere.”