"I'm concerned about Mduduzi; he is not doing enough," Ngubane told KickOff.
"With Mduduzi and Duba, I don't know what their problem is. They are inconsistent, and they are still young.
"Perhaps their lifestyle isn't conducive to their performance. If Mduduzi can focus and take his game seriously, he has the talent to become a star.
"However, he plays well in Cape Town one week and then performs poorly just days later against Richards Bay FC. The same applies to Duba," he added.
"I sincerely hope these two boys will take Kaizer Chiefs and their football seriously. That's my advice to them. You cannot afford to be inconsistent while playing for Kaizer Chiefs."