Boston ? Curt Schilling will miss a start for the first time all season — and just when the Red Sox need him most.

The Boston ace was scratched from his scheduled start in Game 5 of the AL championship series against the New York Yankees because of an injured ankle that was too sore to test Thursday. Manager Terry Francona wouldn’t rule Schilling out for a later start — if the Red Sox make it that far.

“If I can’t pitch without altering my mechanics, then we’re going to have to win a World Series without me,” Schilling said in a call to a local sports talk radio station. “It’s doable, and if you’re a world champion club you’re going to find a way. And I believe we are, and I believe we will.”

Francona penciled Derek Lowe in for the fifth game on Sunday — unless the schedule is pushed back by rain forecast for Game 3 tonight. The Yankees lead the Red Sox 2-0 in the best-of-7 series.

“We need to get to Game 5,” Francona said, “but, yeah, it’s Derek.”

Schilling led the major leagues with 21 wins, and he was second in the AL in ERA and third in strikeouts.

But he lasted just three innings and 58 pitches while allowing six runs in Boston’s 10-7 loss in Tuesday night’s opener; it was his poorest postseason performance since 1993.

Schilling said he had trouble with his balance and pushing off the pitching rubber, which cut down on his pitch speed. He failed to work out Thursday and left Fenway Park without commenting, but he called WEEI-AM to say the injury occurred in his regular-season finale and not in his playoff start against Anaheim.

Red Sox team physician Bill Morgan said the sheath that covers two tendons in Schilling’s right ankle is torn, allowing one of the tendons to slip out of its groove and rub against a bone. Schilling wasn’t uncomfortable while pitching Tuesday because he was injected with a painkiller.

He was expected to be fitted for a brace that would stabilize the ankle. He was planning to test it Thursday during the team’s off-day workout, but he was too sore to try.

“It’s not over. He’s going to continue to try to prepare,” Francona said. “But as far as Sunday goes, he’s not starting.”

Schilling has more than pride at stake. If Boston wins the World Series, it’s worth $15 million to him: a $2 million raise for next season and his 2007 salary of $13 million becomes guaranteed.

With Schilling out, the Red Sox have scheduled Bronson Arroyo, Tim Wakefield and Lowe for Games 3-5. If tonight’s game is rained out, the Red Sox probably would bring back Pedro Martinez for a Game 5 Monday; that would allow him to pitch at Fenway Park instead of Yankee Stadium.

“This team wasn’t built around one player. We’re fine,” Boston first baseman Kevin Millar said.