The Mets offered plenty of reasons for hand-wringing in a 3-1 loss to the Braves Monday night, a defeat that unfolded over 12 excruciating innings. With each frame, the temperature dipped, and the baseball became more difficult to digest.

Suddenly a critical piece because of injuries to the pitching staff, righthander Rafael Montero faltered in his first outing of the season, allowing Matt Kemp’s go-ahead two-run double in the 12th. The Mets have played 21 innings this season and the lineup has scored in just two of them, with Jay Bruce’s homer the only thing separating them from a shutout in Bartolo Colon’s triumphant return to Citi Field.

But something more important emerged from the second game of the season, something far more likely to be telling than a slumping offense or a shellshocked pitcher thrust into a tough spot.

In his first regular season game since elbow surgery, righthander Jacob deGrom looked healthy while tossing six scoreless innings. And as the Mets begin the grind of a long season already two arms down, deGrom’s 98 mph fastballs and wicked sliders seemed to be the most pertinent takeaway of an otherwise forgettable evening.

“That’s the most important thing,” said deGrom, who struck out six. “Like I said all spring, I just want to stay healthy and go out there every fifth day and be able to make my starts.”

Surgery ended deGrom’s season on Sept. 1. He required an operation to correct a nerve issue in his right elbow. But in the spring, his velocity came roaring back, along with his trademark command. He posted a 2.93 in Grapefruit League play with 17 strikeouts in 15 1/3 innings. Of all the Mets’ pitchers, deGrom looked the most ready to begin the season.

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In six innings, deGrom did nothing to dispel that notion. He likely would have gone past the 95 pitch mark had it not been early in the season, when teams generally take it easy on their starters. Besides, his mechanics began to break down, a sign of fatigue.

“I thought he threw the ball very, very well,” Mets manager Terry Collins said.

Colon was no slouch, either. Before signing with the Braves this winter, the 43-year-old had been a cult hero with the Mets, who welcomed him back to Citi Field with a video tribute before the game.

The ageless wonder seemed amped up for the occasion as well. Colon’s fastball on occasion reached 94 mph — a Syndergaardian feat for the soft-tossing strike-thrower. It worked well against the Mets, who managed just one run against their former teammate.

Bruce punished that lone mistake leading off the fifth. One of Colon’s fastballs tailed back over the heart of the plate and Bruce golfed it to right, his shot hitting the orange line on top of the fence before bouncing over.

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Bruce’s homer — the Mets’ first this season — made it 1-0.

Through the season’s first two games, Mets starting pitchers have combined to pitch 12 scoreless innings with 13 strikeouts and just one walk. They have yet to log a victory.

Pitching in relief of deGrom, reliever Hansel Robles brought little command to the proceedings. He took over in the seventh, with the Mets leading 1-0, but promptly surrendered the lead.

Nick Markakis laced a one-out triple to rightfield over the head of Bruce, who made matters worse by taking an odd route to the ball. Robles walked Phillips with the tying run 90 feet away, then allowed Adonis Garcia’s game-tying RBI double.

But Robles wasn’t done, plunking Kurt Suzuki to load the bases, prompting Collins to come from the dugout with the hook. After pitching a scoreless inning on Monday to collect the win, Robles retired just one of the five batters he faced. He left a mess for the lefty Blevins to clean up.

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Working with no margin for error, Blevins struck out pinch-hitter Emilio Bonifacio, then got an inning-ending grounder from Ender Inciarte. With that, Blevins extracted himself from a bases-loaded jam.

The bullpen held firm until Montero faltered in the 12th.

“There’s 160 games to go,” Bruce said, “so you try not to get too high or too low about anything.”