Giancarlo Stanton of the Yankees strikes out to end the eighth...

Giancarlo Stanton of the Yankees strikes out to end the eighth inning against the Red Sox at Yankee Stadium on July 15. Credit: Jim McIsaac

The Yankees haven’t experienced many bad days this season. That’s what happens when you’ve built a double-digit lead in the division to go with baseball’s best record.

Not much to complain about there.

But on the day the Yankees arrived at Citi Field for the Subway Series opener, their first order of business was to put Giancarlo Stanton on the injured list with left Achilles tendinitis.

And it went downhill from that point in a 6-3 loss to the Mets made even worse by the fact they squandered back-to-back homers by Aaron Judge and Anthony Rizzo in the first inning. The Yankees pushed across the only other run on DJ LeMahieu’s fourth-inning groundout in stumbling to their fourth loss in six games.

Without Stanton, Gleyber Torres started at DH and batted cleanup, Matt Carpenter hit behind him and played rightfield. Those two went a combined 0-for-8 and the Yankees as a whole were 0-for-7 with runners in scoring position.

Over in the other dugout, the Mets -- despite the NL East-urgency attached to these two games -- weren’t immediately faced with any crisis, which is usually the default position for this franchise. Their roster was healthy for the most part, and Jacob deGrom was expected to clear his final hurdle with Wednesday’s rehab start at Triple-A Syracuse.

It was a much different story for Boone & Co., as the Yankees manager had to explain the sudden injury to Stanton, who was surprisingly downgraded from “exhaustion” coming out of the All-Star break to this Achilles issue. Two days earlier in Baltimore, Boone sounded confident that Stanton would be fine to start the Subway Series opener. Instead, he didn’t even take the short bus ride to Flushing, and is out indefinitely despite Boone’s optimistic prognosis of a two-week-ish absence.

Apparently, Stanton woke up Monday and requested an MRI that revealed the tendinitis. Boone couldn’t pinpoint when the injury developed, but suggested it was before the All-Star Game, which Stanton homered in to earn the MVP. That might help explain why Stanton was 1-for-13 (.077) with eight strikeouts heading into the break and only made two starts in five games since the second half began with Thursday’s doubleheader in Houston.

“We’ll see where we’re at a week from now,” Boone said. “Hopefully it’s a short stint and turns out, from a timing standpoint, to be something that serves him well and kind of recharges him for the stretch run.”

Adding further insult to the injury, the Mets are probably Stanton’s favorite opponent, as his 37 home runs against them are more than any other team. Stanton also has drilled 23 homers at Citi Field, tops for any visiting player at that venue. That’s not great timing -- however Boone chose to spin it -- and this particular ailment feels like a red flag for Stanton’s usage going forward.

Can the Yankees really feel confident returning him to the outfield on a semi-regular basis? Boone tried to dispel those concerns, saying that Stanton was better off physically as a two-way player. But if that’s true, the Yankees only adopted that philosophy this season, and running around more is never beneficial for a leg injury.

“If we get to that point and it becomes that, so be it,” Boone said. “Maybe he’s avoided other things just because it keeps him more active and more athletic. I think that’s important.”

The Yankees already were doing some serious juggling for this Subway Series by going with Domingo German for Wednesday’s game against the Mets’ acting ace Max Scherzer. With Luis Severino (lat strain) on the IL and Nestor Cortes in need of some innings-preservation over these next two months, the Yankees will have to get creative with the rotation -- and likely need to trade for another front-line starter by next Tuesday’s deadline.

GM Brian Cashman already was in the market for another outfielder, and having to protect Stanton going forward makes that even more of a priority - perhaps enough to make a more serious push for Juan Soto. If Gallo doesn’t figure to on the team after Aug. 2, and Stanton is limited to DH duty, that obviously leaves an available outfield spot.

But that’s not something that has to get done over these 48 hours in Flushing. The Yankees are just focused on plowing through and hosting the roll-over Royals on the other side. As for the Mets, this is Showalter’s first time as a manager in the Subway Series -- it didn’t exist as a regular-season phenomenon during his Bronx tenure -- but he understands the significance of doing more than merely surviving it. Working for The Boss taught him that much.

“I never got that list of who we were OK to lose to,” Showalter said, smiling. “I know it’s important to the fans, so it should be important to us.”

Showalter caught a break for the Subway Series with their Bronx rivals being in some disarray for one of the few times this season. Whether or not that’s enough to derail the Yankees remains to be seen.

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