What has given the Yankees such a home advantage for nine decades — the short right-field porch — is now a detriment.

In the age of ever-expanding shifts, the Yankees’ lack of offensive diversification has made them arguably the most easily defended team in the majors.

“It definitely hurts us,” general manager Brian Cashman admitted.

Before the heavy shifting, the Yankees could fill their lineup with pull-oriented lefty hitters and gain the advantage in their home park of not only plenty of homers, but also when a ball was hit well, singles and doubles.

But put five defenders to the right of second base and try to find a hole, especially at Yankee Stadium. It is one thing to have three infielders and two outfielders shading that way in a roomy right side like at Kauffman Stadium or Turner Field. It still is effective, but not like at Yankee Stadium, which has the least square footage of any right-field area in the majors. Thus, opponents have more defenders arrayed in a small space and, well, good luck to a Yankees lefty pull batter getting a hit that does not land over the fence.

“It definitely has lent to us realizing that a stadium design that used to move us to gravitate to stack lefty hitters and take advantage of our stadium for 81 [home games] has been negated to a significant degree by the shift,” Cashman said. “You have to be aware of it when you acquire talent.”

And this past offseason the Yankees tried to diversify their lineup by getting righty-hitting Starlin Castro and switch-hitting Aaron Hicks, who particularly was obtained to bat righty vs. southpaws.

But what already is entrenched could not be changed. Carlos Beltran, Chase Headley, Brian McCann and Mark Teixeira are among the 30 lefty hitters shifted against most often (all shift stats provided by Baseball Info Solutions). No other team had four, and only the Mariners (Adam Lind, Kyle Seager and Seth Smith) had three — and that did not include Robinson Cano, whose ability to hit line-to-line would have kept him effective at Yankee Stadium even in the shift environment.

We have learned over the past decade, in particular, that batting average is not as valuable as on-base percentage or slugging. Still, it is hard to sustain rallies without hitting success, and the Yanks have been falling in this area pretty much from when they opened the new Yankee Stadium in 2009 (team average of .283) to the .237 they took into this weekend. If that holds, it would be their lowest since 1968, the Year of the Pitcher.

For all the wailing about how the Yanks have not successfully hit lefty pitching — which is true — their bread and butter always has been having their lefty hitters maul righty pitching. But their .234 average in such situations was also the lowest for the team since 1968.

This has been an evolving problem for the Yankees. Since 2010, no team had been shifted on more on balls in play than the 3,677 against the Yankees. The Red Sox were at 3,355 and no other team was within 1,100. That is against righties and lefties and, it should be noted, Teixeira and Alex Rodriguez are among the most heavily shifted righty hitters in the majors. The Yankees’ average in such situations was .193 on grounders and short line drives — only the Phillies were worse.

Lefty Yankees hitters were shifted against on balls in play 3,058 times in that period, the Red Sox 2,931 and no other team was within 1,000.

“It is something we are clearly aware of,” Cashman said. “It will factor in all of our decision making moving forward.”

But what about now? The Yankees avoided adding the kind of pull, flyball-heavy lefty they had in recent seasons such as Eric Chavez, Raul Ibanez, Travis Hafner and Garrett Jones. But the Yanks have to figure out how to win with what remains.

Cashman said his analytics department has done studies that show if you ask players to do what they are not good at — in this case, asking players to bunt or use the whole field more against the shift — they perform worse than even their shifted numbers.

“Some guys are capable of making changes,” Cashman said. “Some guys are not. … We already have seen .265 become .235. We don’t want to see it become .100.”

But it means unless the ball starts going over the wall — especially for multiple runs — it is going to be hard for the Yanks to sustain offense. Think of it like this: If LeBron James only drove right and a team put all its defenders on that side of the court, he is LeBron James, he still would score, but not nearly as efficiently or not nearly enough to make it valuable to his team.

The Yankees’ lefty pull hitters will hit some homers, but probably not enough to justify not taking what the defense is giving them — if only they could take it. So far the middle-of-the-order lefties have not proven regularly capable of changing their approach. That is why we should no longer say a Yankee is hitting in “tough luck.”

He is not hitting in tough luck, he is hitting into the teeth of a defense stacked to thwart even more line drives. It is not tough luck that Headley had the most plate appearances (65) without an extra-base hit going into the weekend. It was hitting it too often where there were fielders.

This is a Yankees curse now. The house built for Ruth — for so long a Yankees advantage — isn’t any longer.