The return of former three-division titleholder Abner Mares was being planned for June 21 as part of a Showtime tripleheader at the StubHub Center in Carson, Calif. However, Mares will not be available for that date because he is changing trainers and needs time to mesh with new cornerman Virgil Hunter.

Out is Clemente Medina, who helped guide Mares to world titles in three weight classes during a 22-month span in which he won a bantamweight belt from Joseph Agbeko in August 2011, a vacant junior featherweight title against Eric Morel in April 2012 and a featherweight belt against Daniel Ponce De Leon last May.

In Mares' first featherweight title defense in August, he suffered a first-round knockout loss in a major upset against Jhonny Gonzalez.

Instead of coming back on June 21, however, Mares (26-1-1, 14 KOs), 28, will instead likely be back later in the summer, manager Frank Espinoza told ESPN.com.

"We had a possible date of June 21 that was offered to us by Golden Boy and we liked that date but Abner made the decision to change trainers and he has to get accustomed to Virgil and spend some time working with him," Espinoza said. "We'll see how Abner's training comes along and I would hope they can get comfortable together and we can get something for him in July."

Espinoza said that Mares just felt it was time to make a change in his corner.

"Obviously, the fighter has to feel comfortable and Abner was thinking it's time to make a change," Espinoza said. "So I totally understand that. At the end of the day, the fighter has to be happy. It was nothing personal [against Medina], just something Abner wanted to do."

Promoter Richard Schaefer said he was informed of the change by matchmaker Eric Gomez and understood Mares' reluctance to fight June 21.

"He wants time to work with Virgil. I can understand that," Schaefer said. "He wants to make sure he is not rushed into a fight before he is comfortable with him."

Hunter is best known for his work as super middleweight champion Andre Ward's trainer, but also works with fighters such as Amir Khan, Andre Berto and Alfredo Angulo.

So Schaefer on Monday finalized the June 21 card without Mares. It will be headlined by welterweight Robert Guerrero against Yoshihiro Kamegai and also include former welterweight titlist Devon Alexander against Jesus Soto Karass and Gary Russell Jr. squaring off with Vasyl Lomachenko for a vacant featherweight world title.