The Cuban, who will corner the Juggernaut upon his return to the ring this weekend against Christian Hammer, was asked for his take on the upcoming heavyweight unification bout between AJ and Usyk.
Joshua, 32, boasts a 24-2 record in his career, but has lost two of his last five fights.
His last bout came in September of last year, when he was beaten by Usyk by unanimous decision.
And Salas has warned him to take heed of Garcia’s advice – or face the consequences if he doesn’t.
Salas told SecondsOut: “I’ve known Robert since he was a junior.
“Robert used to be a good fighter. As a trainer, he showed to me that he’s accomplished.
“The best example was [Marcos] Maidana. Maidana before Robert Garcia, Maidana was nothing. But after Robert Garcia came to Maidana, he changed Maidana.
“As long as AJ can come to the way of Robert Garcia halfway, then Robert can do something with AJ.
“Because sometimes when you have a big name fighter, it’s complicated.
“When you have a big-name fighter and you have one trainer, who tells you that you have to do something, the big-name fighter says, ‘No.’
“He needs people who please the mind. So if he behaves as a fighter with Robert Garcia, I believe Robert Garcia will bring him to the next level.
“When you have any good trainer, no matter who, he’s a good trainer.
“It’s like I say. If he [Joshua] follows Robert, he has a big chance to win. But if he doesn’t follow Robert, he will lose.
“I believe the chance he has got to beat Usyk is to just be more determined in the fight and believe in himself.
“Of course, Usyk is a very good fighter, but very small. He [Joshua] is a big fighter, and they have a saying in boxing. When you have two good fighters, the bigger one should win.
“If the smaller guy wins, it’s because he has more determination.”