PORT ST. LUCIE, Fla. — The relationship between pitchers and catchers ultimately comes down to matters of trust. In practice, that means keeping critical pitches from reaching the backstop.

But when Noah Syndergaard fired a two-strike slider in the dirt in the Mets’ 2-0 loss to the Astros on Friday, Travis d’Arnaud attempted a backhand when he should have used his body to smother the pitch.

It was the kind of bad habit that undermines a catcher’s credibility with pitchers, the kind of mistake that will be scrutinized as d’Arnaud tries to clean up his defense.

The scope of that challenge again took center stage Friday. In addition to the wild pitch, d’Arnaud allowed four stolen bases. He has yet to catch a runner in 12 attempts, with his only caught stealing the result of a pickoff.

“It’s part of the process,” said d’Arnaud, who senses improvement behind the plate in spring training.

The Mets placed a priority on getting d’Arnaud up to speed, hiring third-base coach Glenn Sherlock from the Diamondbacks specifically for his history with catchers. But improvement will not come overnight, as was demonstrated during a rough afternoon in which the Astros stole four bases.

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“It’s certainly something we’ve got to continue to look at,” manager Terry Collins said.

Not all the blame could be pinned on d’Arnaud. Syndergaard allowed a league-high 48 stolen bases last year, and two of the steals Friday came after slow delivery times to the plate. It’s part of the reason that Collins said a “good chance” exists that Rene Rivera will start on Opening Day, not d’Arnaud.

A two-out steal of third base caught the Mets so off guard that Jose Reyes was late to cover the bag. The final steal came on a pitchout, one that should have allowed d’Arnaud to catch the runner. He did not.

It also didn’t help that two of the Astros’ speediest runners reached base. Derek Fisher improved to 10-for-10 on steals with three against Syndergaard and d’Arnaud. Jake Marisnick stole one.

“We know we’re going to face that in the summer,” Collins said. “People are going to run on him. We’ve certainly got to be ready for it.”

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Syndergaard has worked to get better at keeping runners honest. He wants to consistently deliver the ball to the plate in about 1.35 seconds.

“I felt like my times were right where they needed to be,” said Syndergaard, who allowed one run in six innings with five strikeouts.

But as the day wore on, his times to the plate slowed down, rival scouts said. That left Syndergaard and d’Arnaud more vulnerable to trouble on the bases.

Arm strength is not an issue, with rival scouts rating d’Arnaud as league average, perfectly capable of executing better throws. But d’Arnaud has yet to put it all together.

Said d’Arnaud: “For me, it’s about simplifying things, making a good, clean throw, thinking about things that I can control, all those cliches that you’ve heard.”