Yankees pitcher Luke Weaver reacts after the Detroit Tigers' Wenceel...

Yankees pitcher Luke Weaver reacts after the Detroit Tigers' Wenceel Pérez hit a single during the ninth inning of a game Wednesday at Yankee Stadium. Credit: AP/Frank Franklin II

Austin Slater went from mop-up duty on the mound Wednesday to batting leadoff and playing leftfield in Thursday night’s series finale against the Tigers. Not a sentence we expected to be typing with the Yankees in the final 2 1/2 weeks of a playoff push, but it’s been a bizarre season for Aaron Boone & Co., so it looks as if the weirdness is going down to the wire.

Forget that Slater was Boone’s most effective reliever during the first two losses to Detroit, which outscored the Yankees 23-3 while smacking the bullpen around for 18 hits and 19 runs (18 earned) in six innings. The manager is going to need his actual bullpen pieces to perform less like pinatas in the days ahead, or the Yankees can forget about repeating as AL champion come October.

Dating to the dynastic years, the relief corps has always been the heartbeat of the Yankees’ championship teams, anchored by Mariano Rivera. But the runway to the Hall of Fame closer was just as important, because having Rivera was useless without a lead to protect.

This season, however, the bullpen is becoming the Yankees’ Achilles’ heel despite the best efforts by general manager Brian Cashman to strengthen it. Stocking up on relievers at the deadline was Cashman’s signature move long before other teams began to follow a similar strategy. But given the volatile nature of that particular commodity, there’s got to be some luck involved, too.

Based on what has happened since this year’s deadline, Cashman’s good fortune could be nearing an expiration date. Of his three trades for relievers, only David Bednar — now the Yankees’ closer — has worked out, with Camilo Doval sporting a 6.59 ERA in 16 appearances wearing pinstripes and Jake Bird (remember him?) getting demoted to Triple-A Scranton within a week of being traded by the Rockies.

The fallout from that faulty deadline has resulted in the Yankees’ bullpen having a collective 5.57 ERA since Aug. 1, the third-worst in the majors behind the Marlins (5.83) and Rockies (5.75).

For two days against the Tigers, the relievers could barely get anyone out, and it should be especially unsettling that this happened against a team that could be standing between the Yankees and a return trip to the World Series next month. So what makes Boone still have faith in this group?

“Track record, stuff, who they are,” Boone said. “Obviously, we’ve got to get a couple guys on track so we create that depth that we have to have down there. A bullpen ERA in short samples like that can be a little misleading, like when you have a handful of games where it really gets away and it gets blown up.”

Such as the first two games against the Tigers. But going on six weeks isn’t such a tiny snapshot, and the numbers are appalling.

Devin Williams went from being one of baseball’s top closers in Milwaukee to getting booed nightly in the Bronx, pushing him into lower-leverage situations that minimized the damage. Williams was acquired in the offseason, but since the deadline, his 7.11 ERA in 14 games (12 2⁄3 innings) is the worst of the bunch. Doval has been a disaster and even Luke Weaver — nearly untouchable last season after taking over for the struggling Clay Holmes — has thrown some clunkers, including Wednesday night, when he allowed three runs in the ninth.

Of this group, Weaver is the safest bet to stabilize himself in the coming weeks, but it’s hard to trust any of the others besides Bednar (2.70 ERA as a Yankee) and maybe Fernando Cruz, who had a 1.17 ERA in seven appearances before his implosion along with Mark Leiter Jr. (19.64 ERA in his last six games) on Tuesday. The Yankees’ only option at this late date appears to be blind faith.

“I have all the confidence in everybody,” catcher Austin Wells said. “There’s no one else I’d rather have out there. So we’re going to work with what we’ve got and we’re going to get it done.”

Could this relief corps suddenly get its act together down the stretch? It’s possible. Stranger things have happened. Doval throws 100 mph and used to be an All-Star closer with the Giants. Williams had a sub-2.00 ERA in three seasons with the Brewers and a 15.8 K/9 ratio before arriving in New York. Unfortunately for Boone, he’s seen only flashes of those once-elite relievers, and the Yankees have yet to discover a way to make that show up on a more consistent basis.

Their playoff fate depends on it, however. The Yankees have been flushing too many solid starts, as their rotation’s 3.23 ERA is the second-best in the majors (to the Dodgers’ 3.14) since Aug. 1. Will Warren and Carlos Rodon gave the Yankees a solid chance to win both games against the Tigers before the bullpen blowtorched those efforts in historic fashion.

If that continues, October will be over in a blink.

“I have a lot of confidence in their ability and their stuff,” Boone said. “But we’ve got to bring it together. Can we do it? We’re going to find out.”

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