Lebese, 29, is yet to play this season, having scored twice in 15 games in his first campaign with the club last term.
He previously scored 22 goals in 147 matches for Kaizer Chiefs in a spell lasting over a decade at Naturena.
The 33-year-old Laffor, who joined Sundowns in the second half of 2011/12, has played five matches this season across the MTN8 and CAF Champions League, but hasn't featured in the league.
Neither offensive midfielder was used in Downs' Prince Mangosuthu Legacy Cup friendly win over AmaZulu in KwaZulu-Natal on Saturday.
Mngqithi oversaw the Brazilians on the weekend as head coach Pitso Mosimane was in Morocco studying towards his CAF-sponsored Pro License.
"George is not injured, he's trying to work his way in. But you must understand it is very difficult to play for Sundowns, it's very difficult," Mngqithi said.
"From the outside it may look very easy and even the question you are asking about Anthony Laffor, believe me it's very difficult to play for Sundowns.
"If were to give you the position as a coach now and say, 'Okay, I've not been playing Anthony, I've not been playing George'. The position that George must play is the position that 'Mshishi' Themba Zwane is playing. It's the position that Gaston (Sirino) is playing. It's a position that (Jose) Ali Meza is playing. It's a position that 'Pitso', the youngster (Siphelele Mkhulise) is playing. It's where Aubrey Ngoma is playing.
"So do you think these people you want are going to do better than the rest? That is when you start realising that I'm talking too much or I'm not sure what I'm talking about. It's not easy. It is also not easy for us because when we put this one ahead of that one it's because from training we think he's going to do better.
"When he's not playing well it gives an opportunity for you (the media) to ask questions that you are asking. Because when he has not done well, I always say there are many people that are intelligent after the fact.
"When I play this one and he doesn't win and then you (media) say, 'No, if you play that one they could've won'. Yours is not playing any match. When I move to that one and he doesn't win the one that you've chosen, 'No, but this match suited the players we were playing last week. Why did you not play those players?' Your team is never on the field but it is winning every match."
Asked if Lebese is accepting the challenge, Mngqithi said: "It's not about accepting the challenge, he must fight for himself.
"He must fight. Unfortunately he's been a big player, maybe it's very difficult to come to a team that you are not perceived as one of their key players.
"But he's a very good player, believe me he's one of the most technical players that I've worked with. But I just pray that he gets the opportunity one day and he takes it, because it's about getting an opportunity and taking it.
"Because all these players here have had their own opportunities and they don't take it."