Mets pitcher Noah Syndergaard works against the Giants in the first...

Mets pitcher Noah Syndergaard works against the Giants in the first inning on Thursday in San Francisco. Credit: AP/Ben Margot

SAN FRANCISCO — At a time of year when so much of what matters about the 2019 Mets are the developments that will affect the 2020 Mets, an encouraging trend continued to emerge Thursday night: Noah Syndergaard has his slider back.

Syndergaard held the Giants to one run in seven innings, striking out eight and walking one. The Mets eventually lost to San Francisco, 3-2, in 16 innings, after Donovan Solano’s walk-off single against Chris Mazza.

This was Syndergaard’s second start in a row in which he lasted that long, as well as the second in a row in which he used his slider — so good in years past, but missing most of this season — to great effect.

“We saw it last game. It has the depth you want,” manager Mickey Callaway said. “He’s feeling more and more confident with it. He kind of carried that over into today, and it was really good again.”

Consider: In his past two games, Syndergaard has gotten 16 swing-and-misses on his slider. In his previous 14 starts dating to April 15, he had a combined 17 swing-and-misses on that pitch.

“He needs that pitch,” interim pitching coach Phil Regan said recently, “if he’s going to win.”

It’s fair to generalize Regan’s sentiment to the two primary outcomes for Syndergaard’s medium-term future: He needs that pitch to reclaim his status as a high-end starter and help the Mets contend in 2020, or he needs that pitch to restore his trade value the next two-plus months and become a very valuable trade asset for the Mets this offseason.

Either way, Syndergaard finding his slider again would be a key development for the Mets. And it came with an assist from Regan.

When Regan joined the major-league team upon Dave Eiland’s firing last month, Syndergaard was on the injured list with a hamstring strain. They implemented a tiny tweak to Syndergaard’s form in which his feet are closer together as he comes set and begins his delivery. That makes it easier for Syndergaard to get his arm up earlier, which helps the slider’s movement, Regan explained. (And it helps Syndergaard get the ball to the plate quicker — and thus control the running game, long a problem for him.)

“Just real basic stuff,” Regan said. “More of a balanced position.”

After Syndergaard toyed with those changes in a couple of starts before the All-Star break, they seemed to click last weekend in Miami. He returned to the dugout after seven innings, nine strikeouts and no walks and told Regan: “I haven’t had that slider all year.”

The numbers say that, too. Entering play Thursday, opposing hitters hit .269 and slugged .577 against Syndergaard’s sliders. Last year, those numbers were .203 and .233.

On Thursday, Syndergaard had it again as he held the Giants to 1-for-9 with runners in scoring position. He twice stranded a runner after a leadoff triple: Alex Dickerson in the second and Mike Yastrzemski in the seventh.

Tomas Nido, who has caught all four of Syndergaard’s starts since the righthander returned from the IL, said Syndergaard has been trusting his slider — just throwing his slider and letting it go — instead of trying to aim it.

“I wouldn’t want to be on the other side, that’s for sure,” said Nido.

Nido called Syndergaard’s Miami start “a step in the right direction.” Thursday qualified as another.

“When he throws a good slider, it’s (low 90s), that’s a strikeout pitch,” Regan said. “He needs that. He needs that to win. And he needs to command it.”