ATLANTA — With one majestic swing Saturday afternoon, Yoenis Cespedes reminded the Mets of his penchant for seizing the big moment, and underscored how they can ill afford to botch the delicate balance required to keep him healthy.

In his return from hamstring and quad injuries that sidelined him for nearly six weeks, Cespedes blasted a ninth-inning grand slam. In the blink of an eye, he took a tense here-we-go-again late-game nail-biter for the Mets, and turned it into a 6-1 thumping of the Braves in the opener of a doubleheader.

“He showed you today that he’s a difference-maker,” said manager Terry Collins, who must now resist the urge to push the slugger as he eases back into action.

Before the game, Cespedes highlighted the underlying challenge of sidestepping a recurrence of the leg issues that have plagued him his entire career. He acknowledged that despite completing a rehab, he wasn’t sure if he could run at full speed.

It raised questions about whether he was being rushed, an especially sensitive topic during a season in which the Mets’ injury management has been a hot-button issue.

Later, he clarified, emphasizing that he believes he can still contribute without risking another injury.

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“I want to clarify something,” Cespedes said through a translator. “I don’t feel 100 percent but I feel that I am ready to play. I don’t want to injure myself. But knowing that, I wouldn’t go out there to play if I knew I could injure myself, and that’s why I went out there.”

Assistant general manager John Ricco said the Mets were conservative in orchestrating the timing of Cespedes’ return. The rehab included a setback that cost the slugger a few weeks when he felt tightness in his quad after his first rehab game.

“You’re not going to see him immediately go and start playing four or five days in a row,” Ricco said. “He’s going to have to come back and ease back into it.”

Collins admitted it wouldn’t be easy, especially with the Mets trying to claw back into contention after what has been a disappointing start to the season.

The manager insisted that Cespedes would not be available to pinch hit in the nightcap of the doubleheader and mentioned the possibility that he will be benched for Sunday’s series finale.

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“Hell yeah, it’s gonna be hard,” Collins said. “It comes with the territory. It comes with the territory. You can bury me in about two days when he’s not in the lineup.”

Cespedes said he’s fine with the plan — for now.

“For the moment, yes,” Cespedes said. “Starting next week, we’ll see if I’m 100 percent. At that point, I’m not taking days off.”

Of course, Cespedes made it seem that he won’t need much time to get back to full speed. He was prompted to run hard only once, when he ran down a liner in leftfield with no issues. And at the plate, he went 2-for-5 with a homer that changed the game.

Righthander Robert Gsellman tossed 6 2⁄3 shutout innings, giving the Mets a quality start despite having to survive a pair of first-inning errors by shortstop Asdrubal Cabrera. Yet, despite taking a 2-0 lead into the eighth inning, the Mets nearly let it slip away.

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The Braves cut the lead in half on Brandon Phillips’ eighth-inning homer, forcing Collins to summon closer Addison Reed for a five-out save. But in the ninth, Cespedes made sure to provide plenty of breathing room.

With runners on second and third with one out, Braves manager Brian Snitker called for an intentional walk of Michael Conforto to load the bases. Up came Cespedes.

It hardly mattered that he hadn’t played since April 27, or that foul weather in Florida this week kept him from seeing live pitching. Cespedes hammered his fifth career grand slam off reliever Luke Jackson, reasserting why his presence has been irreplaceable.

“We’re going to take care of him,” Collins said. “But he’s ready to play.”