MILWAUKEE — The Mets staked the resurrection of a franchise on their arms — young, powerful, limitless in potential. Through the tumult of the last two seasons, starting pitching emerged as the bedrock, the antidote to a toxic brew of drama, injuries and outsized expectations.

But for the Mets, this season has been little more than a nightly erosion of the dynamo at the center of it all. The unraveling has been unthinkable in both its swiftness and its scope, with rock bottom arriving Saturday night in an 11-4 thrashing at the hands of the Brewers.

A savior when injuries hit a season ago, righthander Robert Gsellman looked helpless at the center of a barrage. With little command of his arsenal, he got hammered for six runs (five earned), nine hits and three walks in four innings-plus.

His departure came under duress in the middle of the Brewers’ eight-run fifth inning. That’s when it became official. At 5.13, the Mets’ starting rotation owns the highest ERA in all of baseball.

“It’s been shocking to me,” manager Terry Collins said. “Shocking.”

This undoing has been a group effort, one that has compromised the Mets’ efforts to reach the postseason for a third straight season.

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Noah Syndergaard’s torn lat will sideline him for at least three months. Matt Harvey’s latest personal crisis has been wrapped around the fact that he’s coming off major surgery and hasn’t been effective since 2015. Zack Wheeler’s return after a two-year absence because of Tommy John surgery has been a slog. Elbow injuries sent Steven Matz and Seth Lugo to the disabled list before Opening Day.

“We still had what we thought was very good pitching here,” Collins said. “We’re having a hard time getting through the fifth inning.”

Jacob deGrom, the new ace by default, owns a 3.80 ERA in seven starts, high for his standards because of spotty command. But at least he’s logged innings. He’s the only member of the rotation to average at least six innings a start.

That’s more than can be said for Gsellman, whose 7.07 ERA normally would have him on a flight to Triple-A Las Vegas. But these are not normal times for the Mets, who already have reached for the scrap heap and added Tommy Milone to fill out a dysfunctional rotation.

Since clawing back to the .500 mark with Wednesday’s win over the Giants, the Mets (16-19) have dropped three straight. Saturday night’s loss snapped a streak of four consecutive series victories.

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There were other culprits, of course.

Shortstop Asdrubal Cabrera cost the Mets a run in the second inning when he committed a fielding error and a throwing error on one grounder, a play that called attention to his diminished defense. Reliever Hansel Robles got lit up for four runs in the fifth, when he entered with the bases loaded and made matters worse, ending a stretch of 14 straight scoreless innings.

But at the center of it all was Gsellman, whose evening was marked by mound meetings, traffic on the bases and pitching coach Dan Warthen getting plenty of face time on television. In seven starts, the righthander has pitched more than five innings once.

“No. I have not,” Gsellman said when asked if he’s ever experienced such a rough stretch. “But it can only look up from here.”

The Mets went ahead 4-2 on Neil Walker’s home run in the fifth, but Gsellman allowed the first four batters in the fifth to reach, capped by Keon Broxton’s RBI single to leftfield.

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Gsellman left the bases loaded with Brewers and Robles allowed them all to score, giving up a two-run single by Orlando Arcia, a two-run double by Jesus Aguilar and a three-run homer by Travis Shaw.

All Collins could do was wonder aloud to Warthen if the hype and expectations had taken a toll. He wondered if the arms had “put too much pressure on themselves.” It’s all he could do to explain how the Mets have come to own the worst starting rotation in baseball.

Said Collins: “I wouldn’t have believed it, not for a second, not with the rotation we came out of spring training with.”