With ace Josh Beckett nursing a sore back, lefty Jon Lester, who has gone from cancer survivor to World Series hero, could wind up being the Opening Day starter for the Boston Red Sox against the Oakland Athletics in Japan.

AP / Elise Amendola

PORT ST. LUCIE, Fla. — When one door closes, another door opens.

The door seemed to close yesterday — or at least start to swing shut — on Josh Beckett’s chances of pitching on Opening Day in Tokyo two weeks from today. Beckett, who was unable to make his scheduled Saturday start because of back spasms, had “a tough night” Sunday, according to manager Terry Francona, and the chances of his pitching in Japan are in question.

“He’s resigned himself to do the right thing and not something silly,” Francona said of Beckett, who stayed back in Fort Myers for treatment. “We’ll be OK; we just have to get [Beckett’s back] calmed down.”

Through that open door stepped Jon Lester.

There was already a shot Lester would take Beckett’s spot at the top of the rotation, even before he took the mound here against the Mets yesterday. Not only did nothing happen to make that less likely, but Lester probably helped his chances.

“He pounded the strike zone. He was aggressive. He looked confident. He was in charge of the game, which is exactly what we’re looking for,” said Francona after Lester had worked four shutout innings, allowing only two hits and one walk with five strikeouts, in a 1-1, 10-inning tie with the Mets. “It was exciting to see.”

“I felt a lot better,” said Lester. “Every start I’m taking a step in the right direction. I’m going to look to build off this. Hopefully I can carry this over to my next start and keep it going.”

Lester earned fame last year as a cancer survivor, to the point that he seemed to be more known for beating cancer than for his pitching ability.

“It was an unbelievable story,” said Francona. “[But] what we’re looking for, and he had a good grasp of this, is once you get on the field, you have to win. For a young kid, he grasps that really well.”

And Lester did his job, downplaying the cancer angle as much as he could. Still, there was no getting around the symbolism of Lester being on the mound when the Sox completed their four-game sweep of the Rockies in the World Series.

“I’m sure it was emotional [for Lester], and it was for me,” said Francona. “But once you take the field, you’ve got to try to win the game. The other hitters, their job is to beat you, regardless of what you’ve gone through.

“He did a really exceptional job of that.”

Lester wasn’t exceptional at the beginning of the exhibition season, as he was touched up for five runs — allowing two hits and four walks — in an inning against the Twins on March 1. He was better on March 5 against the Reds, scattering four hits and allowing a run in three innings, with only one walk and two strikeouts.

But regardless of those results — and even yesterday’s result — the Sox have high hopes for Lester.

“We viewed him as an innings eater for a lot of years,” Francona said before the game. “He’s a big, strong kid. He’s durable and he’s got a good delivery. If he pounds that strike zone, those 100-pitch, five-inning games will turn into the seventh inning because his stuff is plenty good. There are a lot of good things going in his favor.”

The irony yesterday is that Lester’s opponent was Johan Santana . . . the ace for whom he was almost traded last winter. Lester was the centerpiece of one of the packages the Sox were willing to send to the Twins. But the teams never reached agreement on a deal — according to some published reports, Boston pulled Lester off the table — and in the end Minnesota traded Santana to New York.

After the Mets acquired Santana, Lester said he was thrilled to remain with the Red Sox. At this point, the Sox are fortunate, too, especially considering the state of their starting pitching.

They’re not ready to say Beckett won’t make the trip to Japan — “There’s no reason to make that assessment [now]. It doesn’t make sense. We just want to get him better,” said Francona — but a decision could come soon. Team physician Thomas Gill arrived in Florida Sunday, probably to exam Beckett firsthand, and Francona said that spending nearly a full day on a plane ride to the Far East isn’t the best thing for someone with a bad back.

“I can’t imagine a 17-hour flight is good for anybody,” he said.

Lester, for his part, isn’t thinking about stepping into Beckett’s shoes.

“I don’t look at it that way,” he said. “Maybe you just pick him up. You don’t look at it as a different role or a different spot. You get an opportunity to get the ball and you go and pitch, and that’s all you can do.”

But that label — number-one starter — beats cancer survivor every time.

“He’s young and his development got interrupted [by cancer],” said Francona. “Now he’s back and on his way to becoming a pretty good major-league pitcher.”

jmcdonal@projo.com