The 19-year-old came off the bench against Borussia Dortmund in the pulsating 4-3 defeat in Group F and set up a goal for striker Lebo Mothiba.
Letlhaku is fresh from lifting the u/20 Africa Cup of Nations with Amajita in Egypt earlier this month, but the club tournament in the USA is by far the highest level the youngster has played at.
He admits to being overwhelmed in the lead up to Cardoso calling him off the bench at the TQL Stadium.
"Firstly, I was nervous and I was scared of playing against them but the coach came to me and told me that he's thinking of putting me in then, he said I mustn't have pressure, I must just enjoy football. So, I guess that made me feel calm and I played my football," Letlhaku said.
Letlhaku, who was also a substitute in the goalless draw against Fluminense at the Hard Rock Stadium a few days later, adds that facing German giants Dortmund was affirmation that this is the standard Sundowns need to maintain.
"Dortmund is a very big team in the world, so, playing against them was a huge thing for me and one thing happened that when I went in, I went straight to Süle [Niklas] then I was like, this guy is huge. So, that thing made me feel like I have to work hard to be here. So, getting that assist was just a cherry on top of the cake so I must just be grateful," he added.
Cardoso has been vocal about giving players from the Sundowns academy an opportunity at the Club World Cup, and Letlhaku revealed what the Portuguese mentor told him before coming off the bench.
"No, he just said maybe I can give him something so he trusts me then he said Themba [Zwane] is a little bit tired, so maybe I can go in and help the team. Even after the game I was grateful to him, I just went to him and hugged him for giving me this opportunity."