According to the person who advised Justus Sheffield when he was an amateur, the Yankees’ top-rated pitching prospect, by MLB Pipeline’s ranking, is a “bastard” with “no backbone, no integrity whatsoever.”

David Sloane, a longtime agent, represented Sheffield and his brother, Jordan, when the two were drafted. He hasn’t spoken to either since 2014, a result of a legal dispute that resulted in Sloane throwing up his hands and recently deciding not to renew his MLBPA certification, as he first told USA Today. Sloane has been in the industry for decades, representing roughly 125 major and minor league players, most notably Carlos Delgado.

Justus, Jordan and their father, Travis, did not respond to phone calls and text messages from The Post.

When Sheffield was picked in the first round of the 2014 draft by the Cleveland Indians, Sloane negotiated a $1.6 million contract with the understanding, he said, that Sheffield would give him a standard 5 percent commission. That money never came.

Sloane’s agreement with Sheffield was only verbal. He calls that standard practice among agents representing amateur players — a result of NCAA regulations that could see a player declared ineligible — instead waiting until a player signs with a team before officially signing a contract to represent them.

“He used that consideration on my part to stab me in the back,” Sloane told The Post in a phone interview.

After badgering Sheffield’s father, Sloane was given $48,000 — which represented 3 percent. When he next talked to Justus, Sloane accused him of withholding $33,000 and threatened to sue.

Sloane did eventually sue Sheffield, for breach of contract in the state of Arizona, and lost — which Sloane blamed on his lawyers.

Sloane thought that would be the end of it, but a year later, he was notified that the state of Tennessee was fining him $50,740 for violating the Uniform Athlete Agent Act, which requires agents to register with the state when representing athletes.

Sloane managed to get the fine reduced from $50,740 to $10,740 by showing evidence that Jordan Sheffield violated the same law by having an agent while pitching at Vanderbilt. Still, Sloane plans to appeal again, and holds nothing back in his assessment of the Sheffields.

“[Justus] could have easily stood up and said, ‘You know what, Mom and Dad, I understand you’re looking out for what you think are my best interests, but I gave David my word, I believe he did a good job for me, and I’m going to do what I agreed to do,’ ” Sloane said. “But then, he allowed himself to act like a child and let Mommy and Daddy run his life.”