How meaningful is the September performance of the Yankees' youngsters?

 KANSAS CITY, Mo. — What is real and what isn’t as it relates to the month of September has flummoxed those in Major League Baseball for decades.

The oft-used phrase — one used by a cast of hundreds in the sport over the years, from Billy Martin to Branch Rickey — goes something like this: Don’t get fooled by what you see in spring training or September.

The Yankees entered Friday night’s game against the Royals 19-10 since Aug. 28. That record is tied with the AL East champion Orioles for the best in the big leagues in that stretch.

What to make of it?  

“That’s a fun narrative that everyone loves to throw out,” Aaron Boone said last weekend of the age-old saying. “These games matter. We’re playing playoff teams. I find it a little funny that because somebody came up with the cliche of ‘you can’t trust spring training and September,’ everyone just runs with that.”

Boone rarely meets a narrative he doesn’t reflexively push back on — even one such as the spring training/September adage that has its origins from those inside the sport and is not media-driven.  

“I would disagree with that,” he said. “Sure, you have to take it all in and everything doesn’t fall into a certain month, but it’s all part of the important evaluation, especially when you’re talking about young players.”

It is here where the Yankees’ September performance gets interesting.

The impressive record coincides with a youth movement that started, very much at the behest of managing general partner Hal Steinbrenner, in the latter part of August when the Yankees’ playoff hopes essentially went up in smoke after a 2-7 trip through Chicago (White Sox), Miami and Atlanta during a 2-12 stretch that dropped them from 58-53 to 60-65.

Outfield prospect Everson Pereira and infield prospect Oswald Peraza, beaten out by Anthony Volpe for the starting shortstop job in spring training, came in the first wave during the week of Aug. 21. Sept. 1, the traditional call-up date for minor-leaguers, brought catching prospect Austin Wells and the organization’s top position prospect, centerfielder Jasson Dominguez.

But how much of the youth movement has really been a factor in the record?

Not an insignificant amount.

Each of the aforementioned quartet — along with other young players already with the club such as Volpe and Oswaldo Cabrera — have had their moments this month.

It was Dominguez, aka “The Martian,” who made the biggest impact of the group, though it was short-lived. The 20-year-old switch hitter was lost after only eight games with a tear of his right ulnar collateral ligament, which required Tommy John surgery. Dominguez hit just .258 but launched four homers and produced a .980 OPS.

One veteran AL scout who has seen the Yankees this month said of the gaudy record: “To me, it’s been the performance of [Gerrit] Cole and [Aaron] Judge. Those guys [the young players] certainly have shown flashes, Dominguez before [getting hurt] more than any of them. Those guys clearly [have brought] an athleticism and energy that was missing . . . but none of them you would call difference-makers. Cole and Judge have carried them.”

The numbers back that up.

Cole’s shutout on Wednesday night in Toronto put the finishing touches on what likely will be a Cy Young Award season — his first — and completed a September in which he went 3-0 with a 1.03 ERA in five starts. Judge, the reigning AL MVP, entered Friday night hitting .284 with eight homers, 19 RBIs, 25 walks and a 1.082 OPS in 24 games this month.

The AL talent evaluator also gave a nod to the rotation overall, which had a 2.79 ERA in the previous 36 games (since Aug. 20) going into Friday, allowing three or fewer earned runs in 33 of those 36 starts. (That statistic took a big hit when Carlos Rodon was charged with eight runs and didn't record an out as the Royals scored nine runs in the first inning on Friday night.)

Boone has multiple times this month mentioned the benefit of young players getting  a taste of the big leagues in September, which can only help in their development.

“It’s not like, ‘Oh, this guy was great in September, here we go,’ ” Boone said. “But to say it’s not an important part of everything and development and gathering information and gathering thoughts and opinions about [players] — it is important.”

Important, yes. Just how meaningful long-term, however, remains to be seen.