Héctor Neris assured him the blisters that formed along the inside of Seranthony Domínguez’s index finger were nothing to worry about. “That can happen,” Neris told Domínguez as they trained together last winter in the Dominican Republic.

Let the skin heal, Neris said, regrowing stronger, and the next time Domínguez stretched his index finger and middle finger along the sides of the baseball, it would feel more natural.

Neris, a longtime reliever, was right. He’s been throwing a splitter all his career, and Domínguez has taken notice. So, when the Orioles right-hander realized he wanted to add another pitch to his arsenal — something that would bring more versatility to a pitch mix full of glove-side movement — Domínguez looked to Neris for inspiration.

“I know it’s a really good pitch because I watch a lot of baseball games,” Domínguez said. “This year, when I get there [to train in the Dominican Republic], I tell them, ‘I’d like to throw the splitter. Maybe it’s something that can help me, if I can get it.’”

And, sure enough, Domínguez has gotten it.

Once the blisters on his finger mended into calluses better suited for holding the split-finger, Domínguez spent the winter mastering the new offering. He refined it in spring training with oversight from pitching coach Drew French, and he quickly showed how much of a weapon it can be once the regular season arrived.

Domínguez doesn’t throw it often. But what’s apparent in the sample available is amazing: He might have the best splitter in Major League Baseball.

“I think, when he come this season, he maybe was scared to use it, but now, he’s shown me, the last couple times he’s used it, he’s gotten swing and miss,” said Neris, a 2022 World Series champion who now plays for the Los Angeles Angels. “I love that. And he made me happy because he’s making improvement with his work in the offseason, and they look great.”

The training group with which Neris and Domínguez spend their offseason is loaded with major league talent. Beyond the duo of Neris and Domínguez — who met each other with the Philadelphia Phillies — there are Dodgers right-hander Luis García, longtime (and now retired) reliever Joel Peralta and Orioles left-hander Gregory Soto.

In those training sessions, the group shares tips of the trade, such as pitch grips. As Domínguez pondered the splitter, he compared how others in the group held it, and he watched how it came out of their hands.

Domínguez wanted a certain pitch shape — one that was different than anything he previously employed. His four-seamer, slider/sweeper and curveball all have glove-side cut. His two-seam sinker runs back to his arm side, but it averages 97.7 mph. And, while Domínguez used to throw a circle changeup, he found it hard to command. He usually missed below the strike zone.

“My split is more unpredictable. Sometimes it goes straight down. Sometimes it goes down and away,” Domínguez said. “Need something to go away, that’s soft, because the two-seamer is hard. Something soft and away to change the eyes to cause miss or a ground ball, something like that.”

The results are impressive, and they’ve only grown more so as the months have progressed.

Domínguez’s growing confidence in the pitch is apparent by his usage. He threw the splitter 8.8% of the time against left-handed batters in April and 23.9% of the time against lefties in May. That has leaped to a 48.4% usage rate against lefties in June, through Saturday’s games.

Because he had thrown it only 50 times this season (11.1% of his total pitch usage), Domínguez’s splitter run value is 2. When Statcast averages out run value (the expected number of runs a pitch would save based on average) on a scale of 100 pitches, Domínguez’s splitter has a 4.8 run value — the best in the majors.

The raw data supports that.

Only one batter has recorded a hit against Domínguez’s splitter this season, leaving opponents with an .059 batting average against it. The expected slugging percentage against his splitter is .090, which means batters are hardly making hard contact.

Further, Domínguez’s splitter has a whiff rate of 65.5% — the fourth best in baseball. His put-away percentage (the number of two-strike offerings with the splitter that result in strike three) is 38.7%, which is the third highest.

“I saw the video, how good he is, the angle, the velocity,” Neris said. “He can throw it as a strike when he wants it now. He made me happy. When you’re working on something, you want to prove the work. That makes you happy and the guy you’re working with happy also.”

Domínguez can throw his splitter against batters of either handedness, but it’s especially a weapon against lefties. He needed it, because almost everything else in his repertoire worked in on a lefty.

Since the three-batter minimum for pitchers was introduced in 2020, the age of specialist relievers is gone. That has particularly impacted lefties who thrived in left-on-left matchups, but Domínguez felt it too. His arsenal, for much of his career, helped him excel in right-on-right matchups but held him back against lefties.

“I think, historically in the game, a changeup or a split tends to get out the opposite-handed batter, right?” interim manager Tony Mansolino said. “And I think you’re seeing a trend throughout baseball right now where splits and changeups, they’re really showing up right now because of the three-batter minimum. For Seranthony, it was finding a way to handle left-handed hitting.”

The project worked as intended, and while Domínguez’s 4.05 ERA is elevated due to a few blow-up innings, his expected ERA is only 3.01. Should Baltimore be a seller at the trade deadline, Domínguez — armed with his new splitter — could be a coveted option for any bullpen.

“I feel more comfortable,” Domínguez said. “I feel like I have more weapons now to be unpredictable.”