Mets pitcher Jacob deGrom delivers during the first inning against the...

Mets pitcher Jacob deGrom delivers during the first inning against the Marlins during the first game of a doubleheader at Citi Field on Monday. Credit: Jim McIsaac

This wasn’t a Game 7, or a wild-card playoff (stay tuned), or even a must-win late September afternoon. It was early August, No. 112 on the schedule, and the only thing at stake was finally getting back to the .500 plateau, a symbolic mile marker presumably on the path to greater deeds.

For the Mets, however, this was a very big deal. And for those circled games, it’s only appropriate to have Jacob deGrom on the mound, as he was for the opener of Monday’s doubleheader against the Marlins.

Fresh off a Cy Young, and his new $137.5 million extension, deGrom remains the de facto captain of this rotation. So once again, he did what the title requires by providing seven solid innings, striking out eight, in leading the Mets to a 6-2 victory before a cozy but animated crowd at Citi Field.

Maybe this wasn’t vintage deGrom, based on his struggle to settle in early. But he did deliver a two-run single in the fourth inning, marking the 12th time in his career that deGrom drove in as many runs in a game as he allowed. On the whole, it was a winning effort (also trimming his ERA to 2.77) and as the Mets continue their relentless pursuit of a wild-card berth, they’re going to need combined excellence from their rotation, not just the historic greatness their ace delivered last season.

This isn’t a fantasy anymore. The Mets are legitimate playoff contenders now. It’s real. And the primary reason for that is the one that’s always hyped but rarely materializes -- the dazzling potential of the starting staff. After taking most of the season’s first half off, that group is showing up, and has been reinforced by the pre-deadline trade for Marcus Stroman, the former Patchogue-Medford star.

After deGrom’s performance in Monday’s opener, the rotation is 12-3 with a 2.59 ERA since the All-Star break, and these are not merely cameo appearances. The Mets’ starters have thrown at least seven innings in 11 of their 22 second-half games, the most in the majors since the break. Overall, their rotation leads the NL with 35 starts of at least seven innings, second only to the Indians (36).

“Everybody’s been taking the ball and going deep into games,” deGrom said, “so that’s definitely a plus for us.”

Walker Lockett, making his Citi debut in Monday’s second game, failed to keep up as he was gone after just 4 2/3 innings, with the Marlins thumping him for eight hits and four runs. No offense to Lockett, but he’s only passing through.

The Mets are leaning on their front five for this revival, and to help prop up the bullpen, too. Because when it comes to the effectiveness of a relief corps, regardless of its talent level, less is always more. Mickey Callaway really has only three trusted relievers in Seth Lugo, Robert Gsellman and Justin Wilson. In Callaway’s twisted terminology, he’s still calling Edwin Diaz his closer, but in the same breath, mentions how Lugo will also be used in save situations, depending on matchups, usage, etc.

We’re not sure what the thinking was Monday in choosing to call on Diaz with a 6-2 lead in the ninth inning. Obviously, it wasn’t a save opportunity. Our best guess was to use the spot as a relatively low-risk confidence booster for Diaz, but that strategy nearly backfired. The Marlins eventually got the tying run to the on-deck circle, amid boos (and sarcastic cheers) directed at Diaz before he whiffed Jorge Alfaro to seal the win.

The Mets officially have a Diaz Problem. That’s undeniable, and possibly could derail this team’s hard-fought goal of crashing the playoff party. Just as he sabotaged the first half during the Mets’ plummet to 40-50 at the All-Star break. But they couldn’t find an acceptable trade for Diaz at the deadline, and so the Mets need to work around him.

If the rotation can keep pushing deep into games, it gives Callaway more options in the late innings, and the chance to rest critical pieces to fight another day. The key to sustaining the Mets’ resurgence over the next seven weeks (and maybe beyond) lies with the rotation. That’s the reason to believe.

“I think we’re just trusting the stuff more, going right after hitters,” deGrom said. “Every guy is going out there every fifth day and doing a good job. It just makes the next guy want to kind of pick up where he left off.”

And if they follow through on that, the Mets are going to like where this all ends up.