PORT ST. LUCIE — What impressed Carlos Mendoza the most about Jonah Tong‘s five-game big league stint last summer was his ability to wipe the slate clean after tough outings. Tong was an instant success after winning his first Major League decision, but success can be fickle, and it can disappear as quickly as it arrived.

“Looking back, it’s how he was able to bounce back,” the Mets skipper said Wednesday at Clover Park. “He had a tough outing where I don’t think he made it out of the first inning, and he came back the next day and he was just willing to get better and learn from that experience. At this level, you need that.”

Tong, all of 22 years old, came away with the same lesson, understanding why it was important.

“Baseball is a complicated game. It’s my favorite, obviously, but it can be a roller coaster,” Tong said Wednesday after making his first Grapefruit League start against the St. Louis Cardinals at Clover Park. “What I learned last year is how to stay neutral on all of it because you’re going to have days where you’re going to absolutely carve, and other days you’re going to get your teeth kicked in. To be able to stay the same person through it all, is probably the most important thing.”

Tong learned that he not only has to keep a level head from day-to-day, but also from inning-to-inning and maybe even pitch-to-pitch. He debuted a new pitch Wednesday, one he calls a “cutter-slider type thing.” He went to it often, throwing it 18 times. It was working well until the third inning when Nolan Gorman hit one out of the park on a 2-1 count with one out.

The Cardinals went up 3-0, and Tong got the next out before the Mets went to the bullpen. Tong went 2 2/3 innings, allowing three earned runs on three hits, walking one and striking out two with 50 pitches.

“A good learning and development experience here on a day when he didn’t have the changeup,” Mendoza said.

It’s only spring training, so the results aren’t necessarily important, but seeing how the pitch played — even when he couldn’t quite locate it — was still beneficial.

“I hope people can’t read my lips, but I just kept on saying “a ground ball pitch” in the dugout,” Tong said. “It reminded me a lot of the development we had on the slider last year. Just continuing to know that’s definitely a really good option, especially when, like today, the change up wasn’t really there.”

The over-the-top delivery gives Tong power, but it can be more difficult to develop new pitches from such a high angle. To be a starting pitcher in the Major Leagues, you need to be able to throw more than 2-3 pitches, and Tong has never thrown a wide array of pitches. Since the Mets signed the Canadian out of a high school in George, he has always leaned on a high fastball with high velocity, and a changeup.

Hitters exposed this last season in the Major Leagues, helping Tong understand when he needs to use his curveball and his slider. The results he had over his five big league starts last season, (7.71 ERA, 4.31 FIP, 22 strikeouts, nine walks) increased his desire to add more pitches to his arsenal, while sticking with his high arm slot.

“I think it’s just one of those things where you hear a lot of pitchers say that there’s only a certain amount of pitches you can really throw from a certain arm slot; I never really thought it would be harder,” he said. “I think it’s just more something to be comfortable with. So I think as I get more comfortable with things, even like seeing signs of it today, it’s going to be very useful.”

Tong will stay with the Mets this spring instead of pitching for the Canadian team in the World Baseball Classic. The next step for him this spring is to build up his volume and keep working on the “cutter-slider thing.”

“He’s not a fastball-changeup pitcher anymore,” Mendoza said. “Now, he’s got some more weapons that he can use, and today was a perfect example of it.”