"Look, I can beat him [Oosthuizen] everyday of the week but right now I'm in the pinnacle of my career, so there is no reason to going backwards. I have Artur in my mind for now," said the left-handed champion, who will bid for the third defence of his belt in Golden Gloves "No Mercy" tournament.
The 25-year-old former SA, WBC Youth and WBA Pan African champion is rated No 11 by both the WBA and the IBF, and eighth by The Ring Magazine.
Lerena described the 28-year-old Mann, who is rated No 10 by the IBF, as credible opponent.
"He is undefeated in 15 fights, so I can't afford to overlook him. I must take care of business on Saturday and then begin at looking at what lies ahead of me, but Oosthuizen is definitely out of the picture."
Speaking through an interpreter, Mann, a Global Boxing Union Intercontinental and WBO International holder, said: "I would not be here if I did not believe that I will beat Kevin. It does not matter how, the important thing is the positive result."