This was a perfect marriage.

This was the best team in the American League — a club ready to win a championship now — seizing the moment, obtaining the ace it lacked last year to match against Madison Bumgarner, adding the No. 1 starter that seemed the final ingredient to an otherwise title-ready team.

For Johnny Cueto, being acquired by the Royals on July 26 provided an opportunity to burnish his résumé heading to free agency, to tag October stalwart to those dominant numbers he had been accumulating in baseball witness protection in Cincinnati.

But Cueto went off script. After initial excellence, the righty put together a five-start span (0-5, 9.57 ERA, 1.086 OPS against) that jeopardized his status as the unquestioned Game 1 starter, imperiled how much he could make in free agency and created a cottage industry of “what the heck is wrong with Johnny Cueto?”

He made a step Friday night toward dousing some of the hysteria by holding the limping Tigers to one run in seven innings.

Still, there probably is no player who holds this double distinction right now: Most important member of his team for the postseason and player whose near future more impacts the offseason.

The Royals gave up very good prospects for a rental. So Cueto was obtained to start Game 1. Period. Anything less is failure. He was acquired to be the missing piece for a Royals team arguably better than the one that went to World Series Game 7 last year, since Kansas City’s offense is improved while its defense and bullpen remain stellar — though the loss of velocity/effectiveness by Greg Holland is now a huge issue as well.

Cueto and David Price were viewed as the aces traded before the non-waiver deadline. On the day the Royals landed Cueto, July 26, they were an AL-best 21 games over .500 and the Blue Jays were .500 — 10 ¹/₂ games worse than Kansas City. Toronto is just one game behind the Kansas City for the AL’s best record, and because Price was a main reason for the Blue Jay surge, his free agency has only been further spruced.

Cueto has baseball people wondering again about the health of his elbow, among other issues.

From 2011 until his trade, Cueto was second in the majors in ERA (2.51), batting average against (.216) and slugging percentage against (.330) among starters only to Clayton Kershaw. Yes, Cueto was in the NL then, but his home games were in Cincinnati’s bandbox. The bugaboo was fragility, and Cueto always is going to have to work against the perception he breaks easily.

The Royals could have a strong starting group if Cueto has righted himself, the volatile Yordano Ventura does not implode, Edinson Volquez stays steady and Kris Medlen continues to upgrade as he gains distance from Tommy John surgery. That is a lot to ask, but it all begins with Cueto — the piece Kansas City did not have last year.

And it is a reminder that the Mets and their innings limit with Matt Harvey are not the only contender dealing with rotation issues in the waning days of this season. Here are a few others to watch:

1. Yep, Harvey might be getting the most notice, but the Pirates recently skipped ace Gerrit Cole, who already is well beyond his career-high in innings. The Cardinals have spent all season trying to nurse Carlos Martinez (who is approaching 100 innings more than last year) and Michael Wacha (who is 60 innings more than last year). The Astros sent Lance McCullers to the minors to limit his innings, and now he is back as Houston tries to get out of late-season fade.

2. Even at this late date, the Cubs and Dodgers are trying to figure out if they have adequate Nos. 3-4 starters after their dynamic top twos. Chicago almost certainly will have to survive the one-game wild card — probably Jake Arrieta vs. Cole — to use its whole rotation.

Then Jon Lester would open Game 1 of a Division Series against probably the Cardinals. And then? In the past nine games, Arrieta and Lester had combined for three starts, 25 innings and three earned runs while Dan Haren, Kyle Hendricks, Jason Hammel and Travis Wood had combined for six starts, 19 ²/₃ innings and 14 earned runs. Yes, that is cherry picking. But it defines a point that Chicago has a Big Two and then big questions.

The same for the Dodgers, where Zack Greinke and Clayton Kershaw are Arrieta’s main Cy Young competition. The Mets are the Dodgers’ likely Division Series foe, and if they can break through against the Dodgers’ dynamic duo, then what? Alex Wood has been mainly good since his acquisition from the Braves, but the Dodgers already have released Mat Latos, who came in the same three-team deal from Miami. The fragile Brett Anderson probably gets Game 4.

3. The Miracles. When then Toronto ace Marcus Stroman blew out his left knee in spring training, he was deemed out for the season. Now, he is 2-0 with a 3.00 ERA for the everything-going-right Blue Jays. When St. Louis ace Adam Wainwright ruptured his Achilles on April 25, he was ruled out for the season. But he is throwing bullpen sessions and will find out Monday from his doctors if he can make it all the way back this year.