Yankees general manager Brian Cashman at George M. Steinbrenner Field...

Yankees general manager Brian Cashman at George M. Steinbrenner Field in Tampa, Fla., on Feb. 14. Credit: Newsday/Thomas A. Ferrara

 BOSTON

Brian Cashman can talk all he wants about refusing to overpay for pitching help. But if the past week taught the Yankees anything, it’s just how badly they need more of that precious commodity before Wednesday’s trade deadline.

And there’s not an unlimited supply.

The Yankees learned before Sunday night’s 9-6 victory over the Red Sox at Fenway Park that Marcus Stroman, one of their top targets, was headed home to New York — but to Queens rather than the Bronx in a stunning trade by the Mets.

Stroman figured to be a perfect fit for the Yankees, a ground-ball pitcher in these juiced-ball times, with an oversized ego capable of handling baseball’s brightest stage. Factor in his AL East experience, and Stroman seemed well-suited to wear pinstripes.

Now that the former Patchogue-Medford star is taking his talents to Citi Field instead, that’s one fewer available name in an already shrinking market. Maybe the Mets can’t compete with the Yankees on the field this year — the wild card remains a distant goal — but they still managed to deal their crosstown rivals a flashing-neon L by swooping in to snatch Stroman.

So if you envision Madison Bumgarner staying with the resurgent Giants and Trevor Bauer sticking around for the Indians’ playoff push, Cashman’s job suddenly got much tougher.

He still has options, such as Robbie Ray and Matthew Boyd, but we probably can scratch off Noah Syndergaard and Zack Wheeler. We’ll never say never, but the Mets don’t seem to have much of an appetite to deal with the Yankees on these two.

For Cashman, this prolonged search for rotation upgrades — in reality, it kicked off last winter — turned into the perfect storm leading up to the deadline. His starting staff had been an abomination, combining for a 17.67 ERA in a span of 27 innings before Domingo German

was able to momentarily quell the panic with his solid 5 1⁄3-inning stint Sunday, the Yankees’ longest outing in eight starts.  But before anyone throws a party, remember that rotation buzzkill J.A. Happ gets the ball Tuesday against the D-backs.

Also, CC Sabathia was placed on the 10-day injured list because of right knee inflammation, the chronic condition that has haunted him for years. This season was designed to be Sabathia’s farewell tour, but he already has needed one IL stint to regroup — i.e., get a few injections and rest — so having to do it again sounded ominous when Aaron Boone discussed the roster move Sunday.

This could turn out to be more of an extended break, with his effectiveness the rest of the way now in question. Sabathia declined to use his knee problems as an excuse Saturday, but Boone’s response was along the lines of “duh” when it was brought up.

“It’s possible,” Boone said. “I mean, it’s always something I think that affects him. It’s partly why he’s a different pitcher now is because the knee is a real issue for him.”

You don’t need to be an analytics guru to do the math here. Before German’s stabilizing effort, the Yankees’ rotation ERA had soared to 4.77, and now they’re down a starting pitcher.

We get that Cashman likes to maintain a hard-line negotiating stance. Standing pat, however, is not a viable strategy with this starting staff. Not only is Sabathia gone, but the only thing keeping Happ (5.23 ERA) in the rotation is $34 million — and the lack of a replacement.

Cashman didn’t make many mistakes in putting together this first-place team, but Happ is shaping up to be the biggest. The challenge now is finding a way to mitigate the error, and that’s not going to be so easy.

The Mets surrendered two of their best pitching prospects to acquire Stroman — Anthony Kay and Simeon Woods Richardson. Cashman reportedly did not want to part with his own top prospect arm, Deivi Garcia, to get Stroman, and that apparently opened the door for the Mets.

The prices are only going to go up from here. That we’re fairly certain of.

As for what the Yankees will accomplish by the deadline?

“That’s the great unknown,” Boone said.

It better start coming into focus. Fast.