OAKLAND, Calif. — The Yankees are faltering at the worst possible time: In front of their terrifying Ghost of Postseason Future.

How does a one-and-done, loser-goes-home, American League wild-card game here at the Oakland Coliseum sound to Yankees Universe right about now?

That future took one step closer to reality Monday afternoon as the upstart A’s knocked around CC Sabathia and the Yankees 6-3 in front of an announced crowd of 40,546 to draw within 3 ½ games of the Yankees for home-field advantage in that Oct. 3 wild-card game.

“It’s nice to see the Coliseum banging like it was today,” catcher Jonathan Lucroy said. “It’s good for the young guys here to kind of get a taste of that, to get a feel for what the playoffs are like for the guys who haven’t been in them before. Because that’s what it’s going to be like, probably more so in the playoffs.”

“They’re tough,” Aaron Boone said after the game. “It’s obviously a tough lineup to go through with the additions they’ve made to the bullpen and the guys that have really emerged down there. … You know you have to play well to beat these guys.”

The Yankees didn’t play particularly well in their first game against a playoff team since their Fenway Fiasco early last month; they went a respectable 18-9 during their interim Summer Siesta. They finished that stretch with a 3-4 run against the White Sox and Tigers at home, however, and they carried over that blah level of play into this contest.

The A’s, on the other hand? If last year’s Twins, who fell to the Yankees in the wild-card game, appeared “handpicked,” to use Mickey’s term for Rocky’s pre-Clubber Lang opponents in “Rocky III,” then this year’s A’s feel ultra-dangerous … like Clubber Lang. They have emerged these last two and a half months as a club no one wants to face, particularly at home, where they’ll have last licks and are now 42-28.

And while the Yankees, now trailing the Red Sox by 8 ½ games in the AL East, have little choice but to focus on the A’s, the A’s trail the Astros by only 2 ½ games in the AL West.

The A’s slug, like these Yankees — their 191 homers rank second to the Yankees’ 225 in the AL — and they rely on dominant relief pitching, like these Yankees; their 3.26 bullpen ERA places them third, right behind the Yankees’ 3.19 (the Astros are first at 2.94). Yet going back to the whole upstart thing, they don’t feel any sort of heat as does a high-profile, big-spending team like the Yankees.

“This is as good a clubhouse we’ve had since I’ve been here,” said A’s manager Bob Melvin, who landed this job in June 2011.

Oakland’s venerable front office of baseball operations president Billy Beane and general manager David Forst put together yet another contender following a teardown — they made the playoffs every season from 2012 through 2014, then finished under .500 from 2015 through 2017 — with a nucleus of youngsters Matt Chapman (third base), Matt Olson (first base) and Chad Pinder (left field) and complemented that nucleus with a sturdy group of veterans, featuring powerful designated hitter Khris Davis, resurgent second baseman Jed Lowrie (who left after ’14, then returned in 2016) and catcher Lucroy.

“Everyone talks about us being such a young team. We’re really not,” Melvin said. “I think it’s a good mix of guys.”

Until the middle of June, they looked like an ordinary club, the owners of a 34-36 record on June 15. They proceeded to go 10-0-1 in their next 11 series, posting a 27-7 record during that stretch. And that fueled Beane and Forst, with the blessing of ownership, to import a fleet of relievers — including the Mets’ Jeurys Familia and former Yankee Shawn Kelley from the Nationals — to supplement this group, which has lost six starting pitchers (Sean Manaea, Brett Anderson, Andrew Triggs, Kendall Graveman, Paul Blackburn and Jharel Cotton) to the disabled list.

“It’s hard at least for me to put a finger on it, at least for me, on what exactly clicked,” right fielder Stephen Piscotty said. “I think we got on a roll and I think we maybe started to really believe in ourselves and understood we were beating good teams at that time.

“If you do it then, you can do it later. We just kind of continued to keep that mentality and stay hungry.”

They went 3-3 against the Astros in six August games, failing to make up ground directly, and with no more head-to-head matchups against baseball’s defending champions, the A’s no longer control their own AL West destiny. Same goes for getting home-field advantage against the Yankees.

That naturally won’t deter them. They beat the Yankees on Monday thanks to a solid, five-inning start from veteran Trevor Cahill. The 2006 A’s draft pick left in a December 2011 trade, returned as a free agent in March and has enjoyed his best season since 2012. He earned the win backed by a five-run, seven-hit attack against Sabathia in 3 ¹/₃ innings. Melvin utilized five relievers to pick up the last 12 outs, seven of them via the strikeout, with Familia throwing a scoreless eighth. It felt very much like business, rather than some extra-motivational effort, for these A’s.

Asked whether they could send a message to the Yankees this week, Lowrie said, “I think we’ve been sending a message for a while now.”

True. Nevertheless, that message has to feel more personal when it’s delivered firsthand, don’t you think? At the least, the Yankees should resolve to step up and preserve their current future of hosting that wild-card game.