Jacob deGrom of the Mets pitches in the second inning...

Jacob deGrom of the Mets pitches in the second inning against the Braves at Citi Field on Wednesday. Credit: Jim McIsaac

On the same beautiful May evening that perfect baseball weather finally returned to the shores of Flushing Bay, the proverbial black cloud that follows the Mets around suddenly appeared again above Citi Field, locking into its familiar holding pattern Wednesday night by the end of the fourth inning.

The canary in the coal mine was Paul Sewald, who for some inexplicable reason began warming in the bullpen, despite Jacob deGrom — the Mets’ terminator — mowing down the Braves with ease. Six strikeouts, two hits, zero runs and only 46 pitches through four innings.

Moments later, deGrom disappeared down the clubhouse tunnel, the bullpen door opened for Sewald, and this mid-spring nightmare was nearly complete. All that remained was the diagnosis, and shortly afterward, the Mets announced deGrom had suffered a hyperextended right elbow.

Based on the severity, this type of an injury can cost a pitcher up to a month, and maybe more. Obviously, whenever the word “elbow” is mentioned in this context, it’s a disturbing thing for a team to hear. And for deGrom to be stricken, especially with the ragged state of the Mets’ rotation, the news could not have been much worse, far overshadowing the listless 7-0 loss to the Braves, the new leader in the NL East.

Afterward, deGrom already was gone from the clubhouse, dispatched to the hospital for an MRI with the results expected Thursday. But that wasn’t the only question left unanswered. With deGrom suffering the injury at the plate during his third-inning strikeout, on his flailing swing at a 1-and-2 slider, how was he allowed to pitch the top of the fourth?

Mickey Callaway said he was fully aware that deGrom injured his pitching arm somehow with that swing, but let him continue because he “didn’t have any issues throwing.” That prognosis from Dr. Callaway turned out to be premature, however, because deGrom only survived for three more outs, then complained of “soreness in his biceps, so we got him out of there,” the manager said.

By then, it was too late. Previously, deGrom had been pumping fastballs in the 96-97 mph range to overpower the Braves. But in that fourth inning, his velocity was dipping noticeably, and during the last at-bat with Tyler Flowers, deGrom’s final two fastballs came in at 94 mph. Callaway later looked stunned — the manager said he hadn’t seen a pitcher suffer a hyperextended elbow like that before. But the feeling of dread is not all that unusual for this particular franchise.

“I’m sure I won’t sleep very good,” Callaway said. “He’s a big part of our team. But when all is said and done, we can’t sit here worrying about and crying about it. Somebody is going to have to step up if we get some bad news.”

It’s notable that Callaway first mentioned Corey Oswalt, now back at Triple-A Las Vegas, then probably felt obligated to drop Matt Harvey’s name. If Harvey already is distraught over his demotion, imagine how he’d feel if Oswalt jumps him on the depth chart. After listening to Sandy Alderson the other day, however, Harvey’s state of mind is not a big priority for the Mets, and deGrom’s injury isn’t likely to change that.

Before this season began, the Mets’ dreams were built on the potential of their pitching staff, and the possibility of the Fab 5 living up to that faded title. Even with a less-than-spectacular launch by the rotation — ranked 16th with a 4.13 ERA, 14th with a 1.25 WHIP — the Mets rolled to a stirring 11-1 start before cooling off considerably since then.

The unquestioned ace propping up the whole staff wasn’t the intimidating Noah Syndergaard, who had been surprisingly vulnerable on occasion. It was deGrom, the slender assassin, off to perhaps the best start of his career. His 2.06 ERA was the lowest ever in his first six starts, and he trimmed that down to 1.87 — thanks to a current streak of 18 1/3 scoreless innings — before exiting Wednesday night in pain. Now nobody knows for sure when deGrom will be back, or if the Mets can possibly survive his prolonged absence.

“It’s just one of those freaky things,” Todd Frazier said. “You can’t explain it. We’ve just got to keep fighting.”

The Mets won’t give in to the despair, regardless of the deGrom diagnosis. But the frustration, yet again, was impossible to hide.

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