PHILADELPHIA - When the bullpen door swung open Tuesday night, it signaled what the next five games will be about for the Mets.

With the NL East wrapped up, the Mets will be putting their finishing touches on the postseason roster. And in the sixth inning of a 4-3 loss to the Phillies, that meant getting their first look at Jonathon Niese as a reliever.

Niese's first foray in his new role came with the expected bumps. He allowed three hits in 12/3 innings, departing with runners on the corners with two down in the seventh. The lefthander was charged with a run when Erik Goeddel surrendered Aaron Altherr's single, allowing one of the inherited runners to score.

"Today it was good," said Niese, who soon will be asked to pitch on consecutive days. "I was able to get loose quick. I felt great and my stuff was there."

Niese's outing came in relief of Bartolo Colon, whose damage in his five innings was limited to a three-run shot by Darin Ruf in the first.

"If I get the opportunity, I'll continue to pitch," said Colon, who like Niese, also might be destined for the bullpen.

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Lucas Duda hit a pair of homers, a solo shot in the first and a two-run homer with two outs in the ninth, giving him five homers in his last three games.

"He's swinging the bat really well, just when we need him to get hot," manager Terry Collins said.

Meanwhile, catcher Travis d'Arnaud fulfilled a dream, briefly sharing a big-league field with his older brother Chase, who struck out as a Phillies pinch hitter in the seventh.

"I was just laughing," Travis d'Arnaud said of an exchange before the at-bat. "I told him if he got on, he better steal."

Chase got the last laugh, with the Phillies ending the Mets' four-game winning streak. But the result seemed secondary to the Mets' larger goal of preparing for the postseason.

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General manager Sandy Alderson met with Collins and his coaching staff before the game. The group began hashing out the playoff roster.

A few details emerged:

Collins said "the reins will be off" Matt Harvey, who will face no workload restrictions when he's given the ball in the postseason. It remains unclear, however, how often he'll be allowed to pitch.

Alderson said the Mets prefer to use Harvey just once in the best-of-five NLDS against the Dodgers.

As Collins did a few days ago, Alderson hinted at a Game 3 assignment, which would allow the Mets to use Harvey in a critical spot while also limiting him to one start in the series.

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"Basically, he's right where we'd hope he would be," Alderson said. "We're pleased about that. But obviously we'll take into account where he is in the season."

Harvey (1831/3 innings) is scheduled to make one more abbreviated regular-season start, against the Nationals, his final tuneup before starting in the postseason.

Experience won't necessarily be the determining factor as the Mets choose between Colon and rookie Steven Matz for the final spot in the playoff rotation.

"Would you like to have playoff experience?" Alderson said. "Sure, that would be a preference. But would you rather have four horses that throw 95, 96, 98? That would be nice, too. Nothing's ever perfect and I think we have a lot of confidence in our young guys."

Noah Syndergaard's home-road splits will be a "consideration" when the Mets decide how he slots into the playoff rotation, Alderson said. But the GM noted that those differences have lessened over time.

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With home-field advantage against the Dodgers still up in the air, Major League Baseball intends to have the Mets play makeup games if inclement weather over the next five days forces any postponements.

The loss, combined with the Dodgers' NL West-clinching victory late last night in San Francisco, reduced the Mets' lead to one game over the Dodgers for home-field advantage in the NLDS.