MLB

JOHAN STARTS TWINBILL; PEDRO PUSHED BACK

Thanks to last night’s rainout, the Mets will play a doubleheader today. They’re also changing their rotation.

Johan Santana was slated to start last night, so he’ll pitch the first game of the twinbill. Prior to the rainout, Pedro Martinez was scheduled to pitch today, but he’s been pushed back to Monday. Jon Niese pitches Game 2 today in what will be his second major league start.

The Mets said if they didn’t switch Martinez, they would have had two games in next week’s Washington series in which their starter was unknown. As it is now, Oliver Perez will pitch tomorrow, and after Martinez opens the Nationals series on Monday, Mike Pelfrey will pitch Tuesday. Wednesday’s starter figures to be Nelson Figueroa or Brandon Knight, and Santana will pitch again Thursday.

Martinez had no problem with the change and said he will throw today to prepare.

Niese, a 21-year-old lefty, made his major-league debut on Sept. 2 in Milwaukee, giving up five runs in three innings. He’s considered the Mets’ top pitching prospect.

The Braves will pitch Mike Hampton in the opener today, then follow with Jo-Jo Reyes.

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Fans who have tickets for today will be able to see both games of the doubleheader. Fans who had tickets for last night can exchange them for another home game this season, pending availability (check mets.com).

You can try to exchange last night’s ticket for today’s doubleheader, though the Mets said over 50,000 tickets have already been sold for today, so availability is limited.

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Brian Schneider wasn’t slated to play last night because trouble with his left knee. The catcher was examined by a doctor yesterday and it was determined he has bursitis in the knee, along with fluid and swelling. He did not expect the issue to linger into the weekend.

“I’m definitely expecting to play [today],” he said.

Schneider first had trouble with the knee on Tuesday. He said it hurts when he blocks balls behind the plate.

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Billy Wagner told The Post on Thursday that he’s played his “last day as a Met.” If that proves true, Scott Schoeneweis, who visited Wagner on Thursday, said yesterday that it’s not that Wagner doesn’t want to return to Queens.

“He has some inside information that he knows he’s not going to be a Met anymore. It’s not his choice,” Schoeneweis said. “He has some personal things he wants to do, which is understandable, and he wants to close.”

Wagner’s four-year deal expires after next season, though he has a 2010 team option worth $8 million. Essentially, Wagner knows the option won’t be picked up and that the Mets will have a new closer next year.

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Ramon Castro was activated from the disabled list. . . . The Mets lost an MLB coin toss to determine home-field advantage in case of a one-game playoff for the NL East title if they end tied with the Phillies. The game will be played at Citizens Bank Park in Philadelphia.

mark.hale@nypost.com