Jacob deGrom looked like Jacob deGrom, which is what made Paul Sewald warming up in the bottom of the fourth inning startling, which is what made what followed even more unsettling, which is what makes the wait for just what is wrong with the best player on the Mets so anxiety-ridden.

DeGrom had delivered four shutout innings — or in this case, four more shutout innings. His streak was up to 18 ¹/₃ innings. His ERA was down to 1.87.

Those were the numbers as he disappeared into the dugout after the top of the fourth. And now perhaps the most important number is in the offing: When does deGrom reappear? Days? Weeks? Months?

The Mets called what deGrom incurred a hyperextended right elbow. It is not a common injury for pitchers and, indeed, deGrom reported to Mets officials that he hurt it flailing at a 1-2 slider from Sean Newcomb leading off the bottom of the third. Mickey Callaway said his ace explained there was pain, but that the pain was limited to when he hit.

But how could that be known? The third inning was the only time deGrom hit Wednesday night. He went out to pitch one more half inning, delivered yet another 1-2-3 frame and then, Callaway said, informed the manager the discomfort had moved to his biceps. Again, if he felt pain in the most important arm on the team in the bottom of the third, why was he pitching in the top of the fourth?

But he was and he did and now the question is: When will he pitch again?

DeGrom went for an MRI and the Mets announced that the results would not be made public until Thursday.

“I’m sure I won’t sleep very well,” Callaway said. “He’s a big part of our team.”

No one around the club wanted to jump to conclusions without those test results, but here is an assumption that is pretty easy to make: A lot of the Mets season is riding on those results.

For if the season ended today, deGrom would be in the conversation with Arizona’s Patrick Corbin and Washington’s Max Scherzer for the NL Cy Young. But just those words — if the season ended today — feel ominous in this moment.

On a team whose rotation was supposed to be its strength, deGrom was the unquestioned ace, Noah Syndergaard a high-quality No. 2 and then, well, the Mets are experiencing a steep drop. DeGrom and Syndergaard are a combined 5-1 with a 2.46 ERA in 14 starts. The rest of those who have started — Matt Harvey, Steven Matz, Jason Vargas and Zack Wheeler — are 3-6 with a 5.93 ERA in 14 starts.

Remove deGrom and you could see this whole strong Mets opening to the season collapse. As it was, the Mets lost 7-0 to the Braves and Atlanta jumped over New York into first place — the first time the Mets did not hold that position since April 3.

“Obviously, Jake is one of the main cogs on this team,” Jay Bruce said.

He is the main cog. And though all teams suffer injuries of significance during the long season, the Mets losing deGrom for a substantial stretch would be crippling. Callaway mentioned rookie Corey Oswalt or Harvey as potential rotation plug-ins, which at this moment would be like replacing The Rock in an action movie with “Weird Al” Yankovic.

“We hope he just hyperextended it and needs a few days off and that is best for everyone,” Todd Frazier said.

DeGrom was the Mets starter who made it through last year’s injury monsoon that particularly struck the rotation. He was the one player whose performance and reputation stayed at the highest level through the swamp that was the 2017 Mets.

And he was better than ever this year. Adjusting to the launch-angle times, deGrom had mastered another weapon by being able to dominate hitters with fastballs up in the zone. He threw plenty Wednesday night over the first few innings, taming the Braves, making the sight of Sewald beginning to warm up in the bottom of the fourth first a surprise, then a frightening symbol of what might be happening.

DeGrom has a Tommy John surgery in his past. What is in the future will become clearer with the MRI results. To say that the results are vital to who the 2018 Mets can be is to define understatement.

DeGrom is the best player on the team, performing at his zenith. He is a bullpen-saving, run-preventing No. 1 starter. And now, unfortunately for the Mets, their No. 1 concern.