When the Yankees opened the season April 2 at Tampa Bay, there was no question their starting pitcher was going to be Masahiro Tanaka.

He was lit up for seven runs in 2 2⁄3 innings in a 7-3 loss. It began a season of uncertainty for Tanaka, who will bring a 12-11 record and 4.73 ERA into Friday night’s game against the Blue Jays at Rogers Centre.

Nowadays, Tanaka is not the Yankees’ go-to guy. If they host the American League wild-card game on Oct. 3 — the most likely outcome at the moment — Luis Severino almost undoubtedly will get the nod.

Unless . . .

Manager Joe Girardi threw a curveball into the Yankees’ postseason pitching picture when he started Severino, and not Tanaka, against the Twins on Wednesday.

Girardi wasn’t positioning the Yankees for the wild-card game. He was positioning Severino for a possible start on the final day of the regular season if the Yankees still have a chance at the AL East crown.

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The Yankees enter Friday three games behind the Red Sox with 10 to play. If the teams end up tied and in wild-card position, there will be a one-game play-in to decide the division title on Oct. 2 at Yankee Stadium (the Yankees won the season series, 11-8). The loser would get a wild-card berth and would face the other wild-card team the next night.

The Yankees have a 6 1⁄2-game lead over the Twins, who own the second wild-card spot, and a nine-game lead over the Angels and Rangers. The Yankees’ magic numbers are two to clinch a wild-card berth and four to clinch the No. 1 wild card.

But if chaos ensues in the season’s last weekend and Severino is unavailable for the wild-card game, would Girardi have faith in Tanaka to pitch in a winner-take-all showcase? And how would Tanaka feel about it?

“I can’t really talk about what-ifs,” Tanaka said through his translator. “But if Sevy were to pitch that last game of the regular season, someone’s going to have to step up and pitch that wild-card game, so just having that in mind, you always prepare some. If they point to you, then you’re ready to go out there and pitch that game.”

Tanaka pitched in the Yankees’ last postseason appearance, a 3-0 loss to the Astros in the 2015 wild-card game. He allowed two runs — both on solo home runs — in five innings.

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The home run has been a particular problem this year for Tanaka, who has allowed 32 in 165 2⁄3 innings. Entering Thursday, that was tied for fourth-most in the AL.

Tanaka gave up two solo home runs in his most recent outing against Baltimore on Sept. 14. But those were the only runs he allowed in seven innings in a 13-5 victory.

Asked if he thinks his early struggles are over, he said: “Yes. Compared to where I was a couple months back, I do feel like I’ve left that in the past and feel a little bit better on the mound.”

Tanaka has won four of his last five starts and went at least seven innings and gave up three or fewer runs in the wins. But the loss was a stinker: seven runs in four innings on Sept. 8 against the Rangers.

That’s what worries the Yankees — that Tanaka could implode in a win-or-go-home game. But if the AL East race gets tighter, they may have to trust him as if he still were their ace.