Sundowns have been hauled before the PSL Disciplinary Committee and are charged with fielding an ineligible player after Arendse was recalled from the stands before the match to replace Thapelo Morena, who had taken ill just before kick-off.
The Brazilians now face the possibility of being docked points if found guilty of the offence.
"I am a layman in terms of the rules," said Mosimane.
"All I know is we asked the match commissioner if we can do it. I am not saying it's right and I am not saying if you asked you are still right. We asked and we signed. Wits had no problem. Now we are wrong.
"I don't know if the match commissioner knew the rules. I don't know. I don't know if the match commissioner made a mistake. I don't know.
"I am not saying it's right. If it's wrong, it's wrong. But the match commissioner has got the rule book. He can say 'you can't, if you do you get punished'. Do you think we can take that risk? Also, Wits should have protested. They didn't protest."
Despite facing the charge, Mosimane has urged his players to remain focused in their pursuit of defending the league title. Sundowns are now three points behind log leaders Wits with two games in hand.
"We have to focus and try to win," added 'Jingles'.
"Last year we were in the same space with the league where we never knew who was going to get three points, whether Ajax [Cape Town] was in trouble. The poor Ajax went into relegation. They are still there.
"All we knew was to have three points plus because anything can happen. So we got the three points plus."