Aaron Boone has watched Billy McKinney ever since he was an untested Yankees prospect back in 2018.

McKinney briefly debuted with the club, then bounced around for the next five years before finding his way back to the Yankees as an non-roster invitee to spring training this year.

The Yankees’ manager has seen McKinney plenty.

Did he ever anticipate that the outfielder would become as significant as he has?

“I did not, honestly,” Boone said after the Yankees’ 1-0 win over the Rangers on Saturday in The Bronx. “To me, he’s a much better player now than the player I’ve seen in previous years or even spring training.”

McKinney provided the only offense of the day for either team, clubbing a fourth-inning home run against Texas starter Jon Gray.

McKinney worked Gray to a full count, got a down-the-middle fastball and crushed it 423 feet to right.

Billy McKinney belts a game-winning solo homer in the fourth inning of the Yankees’ 1-0 win over the Rangers.
Robert Sabo for NY Post

That was the type of powerful swing, Boone suggested, that might not have come in the past.

“He’s getting into another level of … there’s just a little more maybe killer to the ball, how I would describe it,” Boone said of McKinney, who has smoked four homers in just 15 games. “Where instead of that 380-foot, good fly ball and good at-bat, those are starting to turn into balls in the gap or balls in the seats at this point.”

During a time when nearly the entire offense is floundering without Aaron Judge, McKinney has become perhaps the most reliable contributor over the past two weeks.

The lefty hitter has reached base in all 15 games he has played since the Yankees called him up, marking the longest such season-opening streak by a Yankee since Luke Voit’s 31-game streak in 2019.

McKinney did not seem sold that he is a radically different player than he used to be, but he is happy the production has been there.

“I feel like I’ve always believed in myself,” said McKinney, who is hitting .320. “But it’s nice to put up some results and help the team win.”

As Giancarlo Stanton’s numbers dropped, so did his spot in the batting order.

The Yankees DH/outfielder slipped to fifth in the lineup and went 1-for-3 with a single and strikeout, now in a 4-for-48 funk in his past 14 games.

Looking for a spark to the Yankees’ bats, Boone inserted Harrison Bader in the three-spot and Anthony Rizzo at cleanup, following Jake Bauers and Gleyber Torres hitting at the top of the lineup.

Stanton had not been below fourth in a starting lineup all season. A day prior, Boone said Stanton was “fully healthy.”

“That’s one thing I’ve checked with him and he’s good — he’s looked at me and let me know that, ‘I’m healthy and fine,’ ” Boone said of the streaky slugger.

Stanton is not alone in a Yankees lineup that has been filled with dead spots, but he was the most obvious candidate to step up in Judge’s absence. Instead, Stanton has recorded five total hits (including one home run and one RBI) in his 15 games since Judge injured his toe.

Stanton’s health has progressed to the point that he played right field in Thursday’s loss to the Mariners, but his bat has not reflected that he has felt well.

Giancarlo Stanton singles during the second inning of the Yankees' 1-0 win over the Rangers.
Giancarlo Stanton singles during the second inning of the Yankees’ 1-0 win over the Rangers.
Robert Sabo for NY Post

The Yankees will rely on the pedigree of the 33-year-old prodigious power threat.

“He’s gotta get there [to his potential] at some point,” Boone said.

Anthony Volpe, whose biggest strength this season has been his baserunning, has just two steals since May 14, a span of 36 games.

“He knows what he’s doing. We’re picking our spots,” Boone said of Volpe, who has gone his first 15 attempts without a caught stealing. “There was a stretch in there where he wasn’t on [base] a lot, so he wasn’t getting a lot of opportunities. … That’s going to ebb and flow a little bit just based on game situations.”

In the span, the Yankees shortstop has registered a .244 on-base percentage.