On an unseasonably pleasant afternoon, the Mets began a season of expectations by following a blueprint they established long ago. Ace Noah Syndergaard tossed six shutout innings and the lineup blew open a scoreless game with a six-run seventh as the Mets thumped the Braves, 6-0, on Monday at Citi Field.

“It felt awesome out there, feeling the electricity in the stands, the atmosphere,” Syndergaard said after his first Opening Day assignment. “Walking up the dugout steps, I started getting chills. The roar of the crowd was unbelievable.”

But then the gray clouds gathered. The grounds crew scrambled to tarp the field from oncoming rain and the Mets braced for a storm of a different kind, revealing disconcerting news about their vaunted arms.

Syndergaard was pulled from the game with a blister on top of his middle finger, not a major malady but one that will bump back his next start one day.

Lefthander Steven Matz said he has a strained flexor tendon, a more ominous diagnosis than the elbow inflammation that landed him on the disabled list to begin the year.

But the most troubling development came from Seth Lugo, another pitcher already on the shelf. After the game, Terry Collins said the right hander will be down for “a couple of weeks for sure.”

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Lugo was placed on the DL on Sunday with what the team believed was little more than minor elbow inflammation. But by Monday afternoon, the Mets’ starting pitching depth suddenly looked wobbly, and Lugo said he was headed for a second opinion.

With that, the Mets caught what could be a sneak preview of the conflict that could hover over their remaining 161 games. They reaped the rewards of their talent while fretting about keeping all of it healthy.

Before the blister became an issue, Syndergaard looked like an improved version of the dominant force who took the mound at Citi Field last October for the NL wild-card game. He struck out seven and walked none.

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“It was awesome,” he said. “Kind of felt a little different. I hadn’t been in a competition-based setting in a while.”

As he promised in spring training, Syndergaard threw more nasty two-seam fastballs, a shift in his approach. But he relied on an old standby when he needed it — a 94-mph slider — which he used to whiff Nick Markakis to wiggle out of trouble in the fourth.

Twice, the Braves moved runners to third base with one out. Twice, Syndergaard stranded the runner. He was pulled after 86 pitches and will pitch again Sunday, with Robert Gsellman moved to Saturday.

Syndergaard’s counterpart, righthander Julio Teheran, continued his mastery of the Mets. He fired six scoreless innings of his own, lowering his lifetime ERA against the Mets to 2.60. But when the bullpen took over in the seventh inning, the Mets roared to life, sending 11 men to the plate against four overmatched Braves relievers. It was all to the delight of the 44,384 who jammed Citi Field, the second-largest regular-season crowd in its history.

They roared when Lucas Duda hit a three-run double, when Wilmer Flores sneaked home to barely beat a strong throw from centerfield, when Asdrubal Cabrera triggered it all with a single to make it 1-0.

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“That’s the thing I like about this team,” said Cabrera, who had three hits. “We’re always working together. When you trust your team and know what they can do, a lot of good things are going to come.”

Flores reached on a forceout, swiped second base and scored on Cabrera’s hit. Ender Inciarte made a powerful throw home, and Jeff Kellogg initially called Flores out. But Tyler Flowers caught the ball behind the plate rather than in front of it, which made the difference.

A review showed that Flores got his right foot on the plate before Flowers lowered his mitt. It was the break the Mets needed to pile on.

Curtis Granderson knocked in a run with a sacrifice fly. Jay Bruce worked a walk with the bases loaded. Then Duda turned it into a rout, lining a three-run double off lefty Eric O’Flaherty, with the ball sailing over Enciarte’s head.

“We put some good at-bats together and scored,” manager Terry Collins said. “But Day 1, you hope it sets a big tone.”

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Hansel Robles earned the victory after working the seventh. The Mets improved to an MLB-best 36-20 on Opening Day.

“We were excited to get going today and Noah certainly showed up for us,” Neil Walker said. “And we had that nice seventh inning there to give us the win.”