When Gene Michael was installed as Yankees general manager in late August 1990 – George Steinbrenner’s last official move before serving a 2 1/2-year suspension – he disliked much of what would be the franchise’s worst team in 77 years.

He was particularly appalled, however, by two facets of the offense. Well before Moneyball, he was obsessed with “how easy we make it for pitchers” due to the team’s lack of plate discipline and on-base skills (the Yankees’ on-base percentage was .300, 11 points worse than any other club).

Michael then went to his office window at Yankee Stadium II, pointed to the short right-field porch and said, “We are too damn right-handed.” That season, 68.3 percent of the Yankees’ plate appearances came from right-handed hitters, with righties Roberto Kelly, Steve Sax, Jesse Barfield and Alvaro Espinoza leading the club in at-bats.

Michael committed to build his offense around on-base skills and what always had been vital to Yankees champs – lefty power to capitalize on the stadium’s dimensions. No single move better defined Michael’s philosophy than what was at the time a much-criticized trade: The impatient, righty-hitting Kelly was dealt to the Reds after the 1992 season for the more deliberate, lefty-swinging Paul O’Neill.

In 1993, O’Neill’s first year with the Yankees and the beginning of 25 straight winning seasons, the Yankees got just 49.5 percent of their plate appearances from righties.

They were at 39.7 percent for the 2009 champs and opening of a new stadium and – while the MLB average is annually around 57 percent – the Yankees have not been above 49 percent in the past dozen seasons. Until now.

They went into Wednesday at 58.5 percent – the highest since the season before O’Neill arrived. Unlike the 1990 team, though, which was last in the AL in runs, this one was tied for first.

The simple answer to all of this is: Shift happens.

“We acknowledged that shifts have reduced the right-field advantage we have enjoyed and have played toward historically,” Brian Cashman said. “The shift simply closed down fair territory on the right side of second base. It has reduced the significance of the big hairy monsters we used to go for.”

Yankee Stadium has the smallest area to the right of second in the majors, and more defenders were being pushed there, suffocating batting average/on-base percentage for lefty pull guys such as Brian McCann and Mark Teixeira.

Still, Cashman insists, “this has not been a concerted effort” to go right-handed as a countermeasure to the shifts.

“We are just gravitating to the best players,” he said.

He noted the Yankees gave Gary Sanchez $3 million as an amateur and drafted fellow righty Aaron Judge “before shifts killed offense.”

By last July, Cashman was cognizant that the historic Stadium advantage had shriveled, yet the GM stressed that Gleyber Torres and Clint Frazier – the Yankees’ current top prospects – were obtained for Aroldis Chapman and Andrew Milller, respectively, “because we thought they were the best hitters we could get, not because they are righty.”

The one area in which the Yankees have abandoned the lefty, get-the-ball-in-the-air pull guys they long prioritized is for depth – think of players from recent years such as Eric Chavez, Travis Hafner, Raul Ibanez and Garrett Jones.

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Still, the transition is stunning. The Yankees had finished in the bottom three in the majors in the percentage of plate appearances by righties eight times in the past 12 years and in the bottom 10 in each of the past 14 seasons. They were 30th in 2015 (31.2 percent) and 29th last year (40.1).

This season, they are 17th. Just 41.5 percent of their plate appearances had come from lefties (a rate that is rising with Didi Gregorius back and should rise again if Greg Bird gets healthy).

In spring, this might have been thought of as a real problem, but the Yankees’ .822 OPS by righty batters was the best in the majors, as was their .838 OPS by righty hitters against righty pitchers.

Their two best right-on-right guys are the players who would have caused the most March concern. Aaron Judge (1.124 OPS vs. righties with 11 homers – only Mike Trout’s 12 were more) was inexperienced, possessed a huge strike zone and had a K-frenzy 2016 debut. Starlin Castro, undisciplined in particular with wild swings against righty breaking balls, had a sub-.300 on-base percentage vs. righties in three of the past four seasons. It is .375 this year with a .940 OPS. Since coming off the DL on May 5, Sanchez has a 1.092 OPS vs. righties.

Judge and Matt Holliday, in particular, still use the short porch despite being righties, and when hitting best, Castro goes to the opposite field, as well.

In 1990, Gene Michael knew he had to go left to make the Yankees offense right. There has been a shift over time.