NEW YORK -- On the eve of a series that could determine the outcome of their season, the New York Yankees have lost their best starting pitcher for his next start, and possibly longer.

Right-hander Masahiro Tanaka, who was scheduled to start the final game of the Yankees' three-game series against the Blue Jays in Toronto on Wednesday, has been scratched with a Grade 1 strain of his right hamstring, which he suffered running the bases on Friday night.

Ivan Nova, who had been demoted from the rotation after being shelled for six earned runs in 1 2/3 innings at Yankee Stadium, will make the start in Tanaka's place.

Manager Joe Girardi made the announcement before Sunday night's game against the New York Mets at Citi Field, although he expressed hope that Tanaka (12-7, 3.38) would miss only one start.

Tanaka injured his hamstring in the second inning of Friday's 5-1 loss to the Mets. That he was able to pitch four more innings after being injured led Girardi to believe Tanaka could return to pitch at least once more before the end of the season.

"Maybe it's just something tight," Girardi said. "And he pitched well [after the injury]. But our fear is that if he goes out there on Wednesday, he could hurt it worse. And then you're in a whole lot of trouble."

The Yankees' Masahiro Tanaka has been scratched from his scheduled start Wednesday against Toronto. Ivan Nova will start in his place. Al Bello/Getty Images

Girardi lamented that Tanaka injured himself in the unfamiliar situation of batting; Friday's at-bat was only his sixth plate appearance of the season.

"You get concerned whenever your pitchers have to hit, and you try to do everything you can to keep them from getting hurt," Girardi said. "The one thing you can't prepare for is that sudden burst that they have to make. We tell our guys to take it easy in situations, but I've often said that one of the reasons they are successful is their competitive nature inside of them. He's someone who has hit in the past, and you feel that he's pretty comfortable doing it, and it didn't work out."

Girardi said he noticed Tanaka was slow returning to the mound after batting in the inning.

"He said it was tight, but he felt fine," Girardi said. "We watched his warm-ups carefully, and he kind of signaled to us that he was OK. So you kind of thought, well, maybe it's not much."

Tanaka wound up taking the loss, allowing two runs on five hits, two of them solo home runs, over six innings.

"We waited to see how he felt Saturday," Girardi said. "It didn't worsen overnight. So that leads me to be somewhat optimistic. He didn't wake up Saturday and say it was worse. So he's gonna get a lot of treatment and hopefully get it out of there."

Tanaka underwent an MRI on Saturday that revealed the injury. A Grade 1 strain is the least severe form of an injury; Yankees left-hander Chris Capuano suffered a Grade 2 hamstring strain during spring training and missed the first six weeks of the season.

"I'm not looking at it as something very serious," Tanaka said through an interpreter. "The reason I say that is because I hurt it early in the game and felt strong enough to continue. If I felt that it was something that I could not have handled, then I would not be in that game."

Tanaka, who is pitching with a slight tear in his pitching elbow, spent a month on the disabled list earlier this year with a forearm strain. He has been by far the most effective starter for the Yankees over the second half of the season.

He has beaten the Blue Jays twice in the past month, including a complete-game victory on Aug. 15 in Toronto, and is the most likely starter if the Yankees are forced to play a one-game play-in as a wild-card team. Entering play Sunday night, they are trailing the Blue Jays by three games in the AL East.

"It just feels very unfortunate," Tanaka said of the timing of the injury.

Girardi said Tanaka would not travel to Toronto with the Yankees out of concerns he could further injure himself carrying his own luggage through customs, as is Canadian law.

"We figured it's better for him just to get the treatment at home," Girardi said.

Tanaka is the latest Yankees starter to be felled by injury. Previously, Michael Pineda missed more than a month with a forearm strain, and CC Sabathia missed time with knee problems. Currently, Nathan Eovaldi, who leads the team with 14 wins, is out for the season with right elbow inflammation.

And this is not the first time Tanaka has been sidelined with a hamstring injury; he said that he suffered a more serious hamstring strain while pitching in Japan "about five years ago" and missed a month.

"It's frustrating," Girardi said. "But we have to deal with it, and Nova's going to have to step up for us on Wednesday. Hopefully [Tanaka] will just miss the one turn."