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

Jacob deGrom of the Mets pitches in the first inning against the Giants at Citi Field on Thursday. Credit: Jim McIsaac

Jacob deGrom isn’t unflappable after all. There is some volatility, and on Thursday afternoon at Citi Field, plate umpire Tony Randazzo set off deGrom in the Mets’ 3-1 loss to the Giants,

Pitching with his usual small margin of error for a team that rarely gives him much offensive support, deGrom trailed 1-0 in the fourth inning. Austin Slater was on first with two outs when Nick Hundley came to the plate. DeGrom got ahead 0-and-2 and thought he then caught Hundley looking — but it was called a ball by Randazzo.

DeGrom was upset and wound up walking Hundley. Giants starter Madison Bumgarner, who has 17 career homers but is hitting .129, followed with an RBI double that made it 2-0, and DeGrom barked at the umpire.

“I told him, ‘You can’t miss it,’  ” he said. “ ‘I thought it was a strike and you can’t miss that.’

“Nobody’s perfect out there. Just like we make mistakes, leave balls over the middle that get hit. You can’t expect them to be perfect.

“I was frustrated, but I still got to make a pitch there. I wasn’t able to do it.  I made it 1-2. So I had three more chances to make a pitch and I wasn’t able to do it. I got frustrated, which I probably shouldn’t have. I probably should have kept my cool a little bit and focused on making a pitch. Like I said, I wasn’t able to do that.’’

It was suggested that he got worked up because an additional loss might damage his standing for the Cy Young Award. “It had nothing to do with the Cy Young,’’ deGrom said. “It has to do with being out there competing. You make a pitch you think is a strike, you want it to be a strike, and it wasn’t.’’

Mickey Callaway had no problem with deGrom expressing himself to Randazzo, but he spoke to the bottom line about not losing composure. “It happened so fast, it didn’t get too animated until after the run scored,’’ Callaway said of deGrom’s reaction. “If it starts getting lengthy, yeah, I think I need to go out there. I went and talked to [Randazzo] later on between innings about it. Hey, it’s something that happens. It happens to the best of them, it happens to the worst of them and you gotta go get the next guy out. It’s unfortunate, but that’s how this game is.’’

It was not vintage deGrom, though he allowed only four hits and struck out 10 in six innings to reach double digits for the eighth time this year and 29th in his career. He has 18 straight quality starts, and his streak of allowing fewer than three runs was extended to 23 straight starts, one shy of Dwight Gooden’s franchise record. In the past four games, he has a 0.98 ERA and a 0.98 WHIP and has struck out 41 in 27 2⁄3 innings.

DeGrom ultimately blamed himself, saying, “I just didn’t do a good job of shaking it off and making a pitch, I think that’s all it boils down to.”

“I thought to get through six innings, 10 strikeouts, keep us in the game, he did an unbelievable job,’’ Callaway said. “Probably didn’t have his best fastball command he’s had all year. But he’s such a good pitcher, he’s got other ways to get you out, so his success doesn’t hinge on whether or not he’s got just fastball command.’’

One of the runs off DeGrom (8-8) was unearned, so his major league-leading ERA remained at 1.71. That run scored in the third when Steven Duggar came home on a two-out passed ball by Devin Mesoraco, who left in the fifth with neck stiffness. He was sent for an MRI and results are due Friday, Callaway said.

Bumgarner had pitched 33 consecutive scoreless innings at Citi Field until Todd Frazier homered in the seventh. The Mets then got two runners on with none out, but Kevin Plawecki grounded into a double play and Bumgarner struck out Michael Conforto to end the inning.

Evan Longoria homered off Tyler Bashlor in the eighth to give the Giants a 3-1 lead.

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