NEW YORK - Orlando Hernandez is expected to miss the entire postseason because of a calf injury, and Mets rookie John Maine started Game 1 of the playoffs Wednesday against the Los Angeles Dodgers.

Hernandez has a torn muscle in his right calf and was left off New York’s first-round roster, leaving the NL East champions without two of their top starters. That means erratic left-hander Oliver Perez, who went 3-13 with a 6.55 ERA this season, is slated to start Game 3 or Game 4 in Los Angeles.

“All year long we’ve had guys who picked each other up,” general manager Omar Minaya said. “I’m confident somebody will pick up El Duque.”

With the Mets already missing Pedro Martinez, Hernandez had been scheduled to start the postseason opener. But El Duque got hurt Tuesday while jogging in the outfield, leaving the Mets scrambling to rearrange their rotation.

They turned to Maine, who went 6-5 with a 3.60 ERA in 16 games this season, including 15 starts. The right-hander had not been expected to pitch until later in the series at Los Angeles — if at all.

Had Martinez not been sidelined for the entire postseason by calf and shoulder injuries, Maine might have been left out of the playoff rotation altogether.

Instead, he found himself starting Game 1 at home before a packed house of more than 50,000 fans. And he could wind up pitching twice in the series.

“He’s a tough kid mentally. You know, he doesn’t get intimidated a whole lot by what’s going on,” said Mets pitcher Tom Glavine, who will start Game 2. “He’s a typical young pitcher that’s trying to find his groove in terms of consistency. I mean, he has the stuff to be good and he has the stuff to be successful at this level.”

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Maine lacks the October experience the Mets were counting on with Hernandez, acquired from Arizona in May. El Duque is 9-3 with a 2.55 ERA in 19 career postseason games, including 14 starts, mostly with the Yankees. He also owns four World Series rings, and his teams are 12-3 in postseason series.

“He was obviously very, very distraught. Very upset,” Mets manager Willie Randolph said. “This is his time of year.

“He’s one of the hardest workers on the team, regardless of how old you might think he is. No one lifts as many weights and runs as hard as he does. For him to be out there getting loose, getting ready for the game, to pull a muscle like that is really bizarre. But hey, he feels terrible today. I saw him earlier. He felt like he let us down.”

An MRI on Tuesday revealed Hernandez sustained a tear in his calf.

“That takes time,” Minaya said.

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El Duque lined up along the first base line with his teammates for pregame introductions.

“He’s got a boot on his leg, so not too comfortable getting around,” Randolph said. “We’re going to hold out that, you know, when we get to the World Series that he’ll be able to help us.”

The Mets informed Maine on Tuesday night that there was a good chance he would start Wednesday. After Minaya and Randolph discussed all their options, they decided on Maine late Tuesday night, the GM said.

“He was the perfect guy for it. He was right on turn,” Randolph said.

Hernandez was replaced on the first-round roster by Perez, acquired at the trade deadline with reliever Roberto Hernandez in the deal that sent right fielder Xavier Nady to Pittsburgh.

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Randolph said Perez will start Game 3 or Game 4, if necessary, at Dodger Stadium. Steve Trachsel will pitch the other game.

Trachsel, who was 15-8 with a 4.97 ERA this year, skipped his scheduled outing last weekend to attend to a family matter on the West Coast. He returned to New York late Tuesday afternoon and had a bullpen session.

“We’ll see where Trachsel is,” Randolph said. “We’ll get through the first couple games and then we’ll see where we are.”

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Left-handed reliever Royce Ring also made the team. He had been sent to Florida as part of a taxi squad to stay ready.

The Mets kept 12 pitchers for the first round.

Backup outfielder Michael Tucker was included on the roster, giving the Mets another left-handed bat on the bench. Rookie outfielder Lastings Milledge and outfielder Ricky Ledee were left off. Milledge also was part of the taxi squad in Florida.

New York kept two catchers instead of three: starter Paul Lo Duca and backup Ramon Castro. Mike DiFelice was left off.