It just became too hard. Vito Mielnicki, Jr., the rising 19-year-old star from North Jersey, is 6-foot-1, and his body is still expanding. He had been fighting at around 147 pounds for four years, but the weight was not coming off as easily in recent years.

It was only a matter of time for Mielnicki to move to 154 pounds. His junior middleweight debut will come on the Saturday night, on the undercard of the Errol Spence Jr.-Yordenis Ugas welterweight unification fight from AT&T Stadium in Arlington, Texas, on Showtime Pay-Per-View (9 p.m. ET/6 p.m. PT).

Mielnicki has made a complete recovery from a right biceps tear, which he injured in his Christmas Day victory over Nicholas DeLomba. Looking in the best shape of his life, Mielnicki started the first two rounds at a hellacious pace, then backed off the final seven rounds of the fight.

The reason why?

Mielnicki was essentially fighting with one arm. He had suffered a partial right biceps tear that caused severe pain every time he threw his right.

Mielnicki (10-1, 7 knockouts) will be taking on 30-year-old Dan Karpency (9-4-1, 4 KOs) in an eight-rounder.

“I feel great, my arm feels great, and I’m sparring with my right, and using my right in more different ways than I have in the past,” Mielnicki said. “It feels so natural that I don’t even think about it anymore. I trust it.”

In the last month, Mielnicki has been reunited with someone who goes way back to when Vito Jr. was a baby. Mielnicki is now being trained by Chino Rivas, known for his work with former IBF junior lightweight titlist Tevin Farmer in Cherry Hill, New Jersey.

That’s where Mielnicki wound up, after leaving Joe Goossen, who was committed to Ryan Garcia, in mid camp.

“I’ve known Chino my whole life,” Mielnicki said. “We picked up right from the start and I honestly feel like I’ve learned so much from him in such a short period of time. Everything happens for a reason, and we’ve built such a great bond in a short period of time.

“After this fight, I can’t wait to continue to work with him. I love Joe Goossen. I think the world of Joe Goossen. I will never say a bad thing about Joe. But I wanted more of a commitment from him than he was able to give me. And it wouldn’t be fair to Joe, either.

“I know Joe loves me. That will never change. I deserve what I deserve, and I learned so, so much from Joe. But let’s be real about this, too. I’m sure any trainer in Joe’s shoes would do the same thing Joe is doing in terms of how much time he’s putting in with Ryan.

“At the level Ryan is at, Ryan needs that. I completely understand. Joe and I are good. It didn’t work out and I’m happy with who I’m with right now. I left a great trainer and I’m blessed enough to work with another great trainer in Chino. I will always love Joe Goossen. I just wanted someone fully committed to me.”

Rivas is following up with what Goossen began with Mielnicki anyway, that’s stressing the inside game. Rivas is also improving what Mielnicki does well, working with his legs, and how he uses distance.

Mielnicki is entering this fight with a little something to prove. If only because he wants to compensate for not being 100-percent for the entire fight in his last outing.

“I know Karpency is a tough guy who is a durable opponent, and I want him to be the best version of himself,” Mielnicki said. “I feel so good, even with the craziness in the beginning of camp. I’m in such a good place physically and mentally. I’m ready to go.”

Rivas himself can’t wait to see Mielnicki at 154.

“Vito is a really good kid, and I literally watched him grow up, so this is very easy for me,” Rivas said. “Little Vito is family to me. There’s always been a connection. In his last fight, I was there to see him fight, which comes from love and respect for the kid and his whole family.

“If I had 10 kids who worked like Vito, I would have 10 world champions. What’s crazy is Vito is 19 years old. Think about that, most 19-year-olds want to hang out and chase girls. Vito is all about work. I can’t help myself. I’m really excited about working with this kid.”

When Vito Mielnicki Sr. approached Rivas about working with his son, the decision was easy. Rivas and Vito Jr. began working with Orlando, Florida, and they both flew up to Cherry Hill to work in Rivas’ gym.

Rivas found out quickly how much if a sponge Vito Jr. is.

“Vito makes me work hard,” Rivas said, laughing. “I love Vito like he’s my own son. I keep feeding. Vito is like a computer. He takes in everything. The kid loves boxing. He has a great work ethic. He’s just a pleasure to work with and what you’re going to see on Saturday night is a young man with the experience and composure of someone well beyond his age.

“He’s going to look beautiful. He has the ability to be a superstar.”

Joseph Santoliquito is an award-winning sportswriter who has been working for Ring Magazine/RingTV.com since October 1997 and is the president of the Boxing Writers Association of America. He can be followed on twitter @JSantoliquito.

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