Sundowns booked their spot in the semi-finals after drubbing third-tier side Summerfield 5-0 thanks to a brace by Kermit Erasmus and a goal each by Gaston Sirino, Lesedi Kapinga and Pavol Safranko.
Mngqithi added that the quality at Sundowns makes it difficult for him to know which team is stronger between the second-string and the regular side.
"At Sundowns I find difficult to know which team is stronger and which one is weaker because these boys are very good, if we were to play everybody who deserves to play at once we could even have a 30 v 11 because we've got a very good number of boys who are working very hard to make sure that plant themselves into the team," he told SuperSport TV.
"Yes, when you change the line-up others will say 'what are you doing?' But on the other side you say 'I want to be fresh for the next game because the next match is as important as this one'.
"Looking at a game against Swallows it was a bit heavy, we ran a little bit to make sure we take the three points, then we had to change the team, refresh it and bring these boys because they also deserve an opportunity to play, they are doing well in training.
"The good thing is that we were not starting them, a lot of them having been playing matches either as a substitute or starting so for us it's very important to keep the whole group ready for any eventuality.
"Because you know after this competition we still have the league, we still have the CAF Champions League and if somebody is lagging behind too much, we might end up not having the right players to help us in one of the other competitions.
"So it was important to keep everybody in the right frame of mind and also physically ready because the match itself builds something in the players in terms of minutes."