Sundowns resumed Absa Premiership action with a 3-3 draw against Usuthu on Sunday afternoon.
Pieterse was a notable talking point following the draw, but Mosiane was quick to defend his shot-stopper.
"I’ve seen Petr Čech make a mistake. I’ve seen (David) De Gea make a bigger mistake than that. I’ve seen Denis Onyango make a mistake. I’ve seen Itumeleng Khune make a mistake. So it’s okay, he (Pieterse) is allowed to make a mistake," he told SuperSport.
“Who doesn’t make mistakes? Unfortunately, when a goalkeeper makes a mistake, he is the last man so we should understand it.”
Mosimane will now shift his focus to Wednesday's league encounter against Golden Arrows.
“We have to try again against arrows,” he added.
“If you score three goals away then you should be able to win, but that was not the case against AmaZulu.
“I’m glad we got to play football. Cavin Johnson plays football well, Benni McCarthy plays well, Steve Khomphela is playing well, it’s good for the local coaches.
“The product is good and that’s what we need to watch, it’s the kind of game we like.
“Unfortunately, the three goals conceded, I wonder if I can analyse because the ball was falling off of this one and that one and they score, but it’s okay. I’m happy my team scored three goals.”