The Colombian striker will miss the Nedbank Cup last 32 clash against Highlands Park on Sunday after picking up his fourth yellow card of the season against SuperSport United on Tuesday.
The two sides clash at Makhulong Stadium in Tembisa on Sunday, with kick-off set for 15h00.
“Our flow, of course, is going to be a little bit disturbed because, when we don't have Leo to play with Khama [Billiat] and Keagan [Dolly], it [cancels out] that CBD thing,” Mosimane says.
“But we will see who will play in the position of Castro, whether Ngele, Mashego or Malajila comes in, we'll see and we'll make a decision as to which one of those three will take the position.
“We'll struggle a bit, obviously, we don't expect them to flow because they haven't gotten regular game-time.”
Meanwhile, Mosimane has admitted to employing the tactic of mind games against Highlands Park coach Allan Freese.
The Lions of the North are currently third on the National First Division standing, trailing leaders Baroka FC by just four points with nine games to go, and could secure a spot in the promotional play-offs if they maintain their position.
On the other hand, Sundowns hold the Aces as they lead the Absa Premiership standings by 10 points to their closest rivals, and face very little pressure in this weekend's cup tie.
Mosimane says, albeit jokingly, he has already shared a word of advice to the former Platinum Stars mentor.
“They are lying third on the log which shows they are almost a team in the PSL," says 'Jingles'.
“I was just joking to him and asking if he wants to beat Sundowns, and keeps going until he loses in the quarter-final after accumulating a lot of yellow cards and injuries, and ends up not even competing for this position [in the League].
“I said: ‘What do you want? It's good to eliminate Sundowns and there's headlines, but what's your objective when you go into the First Division as a top coach? It's to take the team to promotion.’
“I was joking with him and he understands what I'm saying, you know, mind games - trying to make him play a weakened team and not his strongest players.”