The prism is the playoffs.

Everything the Yankees do now is with an eye on October. Even with an inexcusable 4-3 loss Tuesday night to the Reds — fueled by a ninth-inning meltdown by Clay Holmes — the Yankees lead the AL East by 14 games. With the Orioles no longer doormats, the top-to-bottom quality of the division will almost certainly keep any club from building enough wins to overtake the Yankees.

So the agenda is, of course, win the division, hold off the Astros for the AL’s best record and focus on setting up for October success.

Which leads to this question: Do the Yankees have a trustworthy playoff rotation?

This squad is now often compared to the sainted 1998 Yankees. But that team had not only a strong rotation, but one that proved unflappable in the postseason. For me, Orlando Hernandez has been the best big-game starter of this three-decade run of Yankee excellence. David Cone is not far behind. Keeping David Wells’ attention was not always simple, but it was not a problem in October. And when he was not tipping pitches and especially over time, Andy Pettitte learned to manage a postseason game expertly.

Do you see the same potential for the current group?

“Our guys are capable of putting up quality starts in the postseason,” said pitching coach Matt Blake. “I think that the question is, if we get to a postseason, which guys are the ones taking the start and which go to the pen? We’re looking at a handful of guys that are probably coming up on innings that they haven’t been at in the last few years. So where’s the fatigue factor in terms of how we adjust in the second half?”

The big concerns are Luis Severino, who hardly pitched the previous three seasons, and Nestor Cortes, who is 4 ¹/₃ innings shy of his major league career high already.

This will have to be weighed before the Aug. 2 trade deadline. Cincinnati’s Luis Castillo (scheduled to start against the Yankees on Thursday) and Oakland’s Frankie Montas currently highlight the top of the starting market. But there is volatility. For example, do the Giants continue to stumble and put Carlos Rodon in play?

Part of calculating how much prospect collateral (if any) to expend on a starter is determining which Yankee rotation to believe in — the one from the beginning of the season to June 5 or the one since then? The overall positive is health. No Yankee starter has gone to the injured list this year.

Still, in the first 54 games — exactly one-third of the season — the Yankees rotation led the majors in ERA, all three slash lines, walk percentage and was second in strikeout percentage. In the subsequent 32 games going into Tuesday, its 4.42 ERA was 18th in the majors and all the other categories were noticeably down too.

And there was one powerful culprit — the home run. The Yankees gave up the fewest in the first 54 games and the most in the next 32. Gerrit Cole, Cortes and Jameson Taillon have been particularly slugged. Taillon had gone from a .353 slugging percentage against (through June 5) to a MLB-worst .608 since.

Cole had no long-ball issues against the meek Reds. He delivered seven shutout innings, ramping up when most necessary. He held Cincinnati to one hit in 12 at-bats with men on base, including striking out the side after a leadoff single in his seventh and final inning. His 113th and last pitch was his hardest of the game, 100.5 mph to whiff Mike Moustakas.

But the Yankees’ offense was kept inside the park and squandered opportunities all game to position a loss when Holmes had his worst outing as a Yankee.

For the Yankees the homer issue was part of a league-wide matter — there was a homer hit every 36.6 plate appearances through June 5 and 31.9 afterward. There is some regression to the mean — the Yanks were not going to deflate slugging like that all year. But Blake, Taillon and catchers Kyle Higashioka and Jose Trevino all said pitching mistakes were more frequent — just leaving too many pitches in delectable hot zones. After doing so well at fixating on limiting slugging by particularly attacking the bottom of the zone, opponents also have gone to school on the Yankees; foes seem to be adapting to Cortes’ cutter, for example.

So do the Yankee starters have to change again? Is this the first sign of fatigue? Are outside upgrades necessary? Or is this all just a blip in a magical season?

“I wouldn’t discredit this group; they’ve been good all year,” Trevino said. “They’re gonna be fine. There’s no red flags for us.”

But the Yanks have been so good that this is their season now, hunting the red flags that could undo October — is Holmes suddenly one? So, do they believe this is a postseason rotation that will go into Fenway and/or Houston, deflate homers and rise to the playoff challenge? The Yanks have played so well to this point to focus on that big picture, too.