The sound of the baseball jumping off the bat of Marlins slugger Giancarlo Stanton was loud, crisp and emphatic.

Surely everyone at Citi Field knew where this second-inning blast was headed — toward the seats in left-centerfield, well beyond the wall. It was the type of home run that scouts commonly refer to as a no-doubter.

Truth be told, though, Steven Matz still turned to watch the ball’s flight and see where it landed. The dejected look on his face told the story. This was anything but good.

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Matz’s next move was to walk off the mound, head down, his 2016 season debut finished after he recorded only five outs.

To call this abbreviated outing a far cry from everyone’s expectations — Matz’s included — would be an understatement. The Long Island lefthander allowed seven runs in 1 2⁄3 innings Monday night as the Mets (2-4) fell to the Marlins, 10-3, before 24,320 for their third straight loss.

“I wasn’t very good,” Matz said. “It’s about wins and losses. It’s just a loss. I’ll work as hard as I can to get back on track.”

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The shocking part of the first regular-season loss of Matz’s major-league career was just how quickly things went south.

All seven runs that Matz allowed were scored during a second inning in which he threw 40 pitches — nearly half of the soft pitch count of 90 that the Mets had in mind.

“One of the things we certainly try to stress here is when it’s not working, you’ve still got to pitch,” Terry Collins said. “He’s got to use his pitches.”

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Pitching for the first time since an exhibition game in Las Vegas on April 1, Matz breezed through the first inning. He was not affected by the umpires reversing a third-out call at first base on replay, which put a runner on with the always dangerous Stanton at bat. Matz struck out Stanton swinging on a 2-and-2 sinker clocked at 94 mph.

Considering all the optimism surrounding the 25-year-old’s first full season — he was 4-0 with a 2.27 ERA in six starts last year — Stanton’s swing-and-miss had the look and feel that this would be Matz’s night. But that proved to be his only highlight.

“He came out in the first inning and made every pitch he had to make,” Collins said. “In the second inning, he just lost the feel for every pitch. He didn’t make any pitch he had to.”

Matz began the second by issuing a four-pitch walk to Martin Prado and never recovered. After another walk and a single loaded the bases, Adeiny Hechavarria hit a hard grounder that ricocheted off the third-base bag into leftfield, allowing the first two runs to score. A sacrifice bunt put runners on second and third and Dee Gordon’s infield single made it 3-0.

After Gordon stole second to put runners on second and third, Matz got Marcell Ozuna to pop out for the second out. But then he hung a 2-and-2 curveball that Christian Yelich lined to center for a two-run single. Stanton followed with his moonshot, and Collins had seen enough.

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Matz was coming off an uneven spring training in which he expressed frustration at times about his struggles to get outs. But he pitched better late in camp, and Collins hoped to see Matz build off that Monday night.

Now the manager hopes his young pitcher has a short memory.

Said Collins, “We know he’s a lot better than that.”