Anyone can do the math, and that would include members of the Yankees rotation. With a limited number of slots available for postseason starts, two of them locks for Masahiro Tanaka and James Paxton and a third likely reserved for Luis Severino if he is healthy when October rolls around, that leaves one opening.

J.A. Happ intends to fill it.

“I came here to be a part of the postseason — a big part of it,” the 36-year-old lefty said after the Yankees struck for back-to-back solo homers in the bottom of the ninth to steal a 5-4 game from the A’s in The Bronx on Sunday afternoon. “That’s my plan. That’s how I feel about that.”

Happ, who took the ball and the defeat at Fenway in Game 1 of the ALDS last year after his acquisition from Toronto shortly before last season’s deadline, may not perceive his outings as auditions for an assignment this postseason, but his recent work sure hadn’t put him in prime position to get the call. He hadn’t made it past the fifth inning of any of his past five starts and had turned in only one quality start of his past 13 dating to June 6. He had allowed 39 hits in 34 innings with a 7.68 ERA in seven starts since July 24.

So, playoffs? Playoffs?

And what about 17-game winner Domingo German?

But then came Sunday, when Happ was the pitcher the Yankees had envisioned and expected, allowing just one hit, a second-inning single, while pitching six shutout innings. Happ was in command pretty much throughout, and when he wasn’t for a brief spell in the fifth when he walked the first two batters and went to 3-0 on Jurickson Profar, he restored order by striking out Profar looking and getting the next two without fuss.

“It was really encouraging,” Boone said. “Obviously no runs in six innings against a really good offensive club, that’s one thing, but the couple of times it had a chance to get a little away from him he buckled in and made some great pitches.

“So big pitches when we needed them to right the ship. I thought he was really good. Six innings and no runs speaks for itself.”

This was a 40-man day for the Yankees, who recalled three pitchers and Clint Frazier on the first day of the last season of September roster-bloating. Having gone through seven relief pitchers Saturday, the team needed some length from Happ. They also needed a pitcher to give the Yankees time to think of something, being as they did as little against Oakland starter Sean Manaea as the A’s did against Happ.

It was the promoted Ryan Dull who allowed three runs in the seventh and then the promoted Chance Adams to surrender another run in the eighth. Four-nothing, the other guys into the bottom of the eighth.

Four-nothing? That’s nothing.

“We believe in ourselves to the very end,” Brett Gardner said. “Even if we’re down four.”

Four became one following a three-run eighth inning. One became none when Gardner, who’d fanned in his first three at-bats and was in a 1-for-17 rut, struck a home run to right to lead off the ninth. Mike Ford, pinch hitting for Frazier in a lefty-righty matchup against Oakland closer Liam Hendriks, followed with a blow into the home team bullpen in right-center to end it.

“It was just lots of fun,” said Gardner. “No moment is too big for anyone.”

The Yankees turned a pair of infield double plays in the first two innings, the second notable for Didi Gregorius’ range at short and Gleyber Torres’ howitzer to first to complete it. “Special,” is what it was called by Boone, whose team turned three double plays in Saturday’s 4-3 victory that the Yankees tied on an Aaron Judge home run in the eighth before DJ LeMahieu won it leading off the 11th.

Two days, five double plays, two walk-offs, the best record in MLB at 90-48, the fastest the Yankees have reached 90 wins since 1998 and the second-fastest since 1964. That encompasses a lot of pennant winners and World Series champions. But if this team doesn’t get to 11 wins in the postseason, no one will remember, let alone care, about the regular-season record.

That’s why the Yankees need to nail down their pitching. Severino threw 33 pitches for Triple-A Scranton/Wilkes-Barre in his first game action of the year on Sunday. There’s a way to go from there to here. So Severino in for the postseason rotation? German? CC Sabathia?

Happ had something to say about that Sunday. And with more than his words.