JUPITER, Fla. — Steven Matz hadn’t pitched since Aug. 14, his season ended by shoulder soreness and a painful bone spur in his left elbow that required surgery.

But Matz said neither his shoulder nor his elbow was on his mind when he made his Grapefruit League debut against the Marlins on Monday. The former Ward Melville star was too busy working on a new pet project to think about the injuries that have been a blight on his career.

Instead, Matz used his two-inning stint to clean up a part of his game that opposing baserunners have exploited. Throughout spring training, the lefty has worked to get better at keeping runners honest.

“I’m just trying to get the feel back for everything,” Matz said. “I’m reading guys at first base out of my stretch. I’m actually glad I was able to work with some runners on there.”

While Noah Syndergaard’s troubles with runners on base generated the most buzz last season, Matz proved nearly as bothered by base-stealers. Opponents swiped 20 bases against him, tied for the ninth most in baseball.

Matz allowed three hits in two scoreless innings. After averaging 94 mph last season, his fastball sat at 92, about the expected range for the first start of the Grapefruit League season.

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“I definitely had some nerves going into it just because it’s been a long time since I’ve been out there,” Matz said. “It felt kind of awkward for a second. But once I started throwing, it felt good.”

The three hits Matz gave up were a slow roller up the middle by Destin Hood, a bloop single to shallow right by Giancarlo Stanton and a jam shot to right by Brian Anderson. But Matz didn’t mind. The traffic on the bases gave him an opportunity to practice his new move.

It has been an emphasis during live batting practice and other side work leading up to his exhibition debut. By holding his leg up for slightly longer in his delivery, Matz allows himself to throw to first if a runner is going.

“Well, it will be huge for him,” manager Terry Collins said after an 8-2 win over the Marlins. “I like what they’re doing with him. Instead of trying to speed up his delivery, they’ve taught him to hang in the air and be able to read the runner. And actually in practice, he’s done very, very well at it. So I think it’s something that will help him and it’s good to get him back out there again and get him moving towards the season.”

Matz tried the move last season — and balked. But he intends to keep working on it, resurrecting lessons that former Mets lefty Scott Rice gave about holding runners when they were teammates at Triple-A Las Vegas.

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Said Matz: “I kind of remembered that, and I thought I’d give it a try this spring.”

Rice, who pitched for the Mets in 2013 and 2014, had been encouraged by pitching coach Dan Warthen to help Matz with ideas on how to be more mindful of runners.

“Matz and I talked about runners going first move on lefties,” Rice said Monday. “So if your first movement is slow, you will be able to read better. You can peel your front foot slowly off the ground or move your glove first before you even lift your leg. Basically, you always want to be quick to the plate, but your first movement should be slow.”

Pitching from the windup allows Matz to collect himself at the top of his motion. With runners on base from the stretch, he tends to rush. But with the new move, he said he feels more comfortable in the stretch.

Matz’s only strikeout came against his first batter, as he froze J.T. Realmuto with a fastball that caught the inside corner.

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He was pulled after 39 pitches, officially completing the Mets’ starting rotation rollout. After Tuesday’s scheduled off day, Opening Day starter Syndergaard will pitch on Wednesday, beginning the Mets’ second turn through the rotation.

With David Lennon