CINCINNATI - The Mets' pitching plans for the postseason came into sharper focus Sunday when manager Terry Collins said lefthander Jonathon Niese will begin working out of the bullpen.

Collins said Matt Harvey, Jacob deGrom and Noah Syndergaard will be in the rotation, though he did not specify in what order. The fourth spot will go to Long Island lefthander Steven Matz or righthander Bartolo Colon, but even that matter appears mostly settled.

Collins reiterated that he doesn't consider Matz an option to pitch in relief because of his past health issues. "I don't think it's a good idea," he said.

So Matz appears lined up to start, allowing the Mets to use their four youngest and talented arms against the Dodgers.

Even if Colon -- who will start this week against the Phillies -- winds up in the bullpen, Collins said he likely won't need a transition period like Niese, who could give the Mets the kind of lefty reliever they have sought all season.

"It's big, being in the bullpen, coming in in crucial situations," said Niese, who likely would have been squeezed out of the rotation before he approached Collins with the bullpen idea.

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"If I can be that guy that can get a lefty out, I'm definitely glad to be that guy," said Niese, whose second-half slide diminished his chances of remaining in the rotation.

Niese has only one major-league relief appearance, in 2011. Logan Verrett will start in his place Thursday in Philadelphia.

Harvey will start against the Nationals on Saturday. He likely will be capped at three or four innings although, according to Collins, Harvey wants to reach 75 to 80 pitches to stay sharp for the postseason.

Collins said it's unclear whether Harvey will pitch more than once in the NLDS, and he hinted that a Game 3 assignment "might be a perfect spot" for the righthander. Collins called the third game of a five-game series "pivotal."

The assignment would limit Harvey to one start in the series, with the Mets still monitoring his workload.

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Harvey lobbied to approach the 100-pitch mark during Saturday's clincher, even pushing to begin the seventh inning when Collins wanted to pull him.

Harvey allowed two runs in 62/3 innings against the Reds. He was allowed to throw 97 pitches before being pulled from his longest outing since Sept. 2.