Sundowns beat ASEC 3-1 in the first leg played at Loftus Stadium on Friday night as Mosa Lebusa, Themba Zwane and Emiliano Tade scored the goals for the home side to record their second win of the group stage.
The Brazilians now need to win their next match to guarantee a place in the quarter-finals but despite Saturday's victory, Mosimane was left disappointed with a number of missed opportunities.
"Against Wydad [Casablanca] we should have scored three goals. Sometimes Sundowns can give you heart attack," said Mosimane.
"There was a miss by [Gaston] Sirino, miss by Anthony Laffor, hit post, you can count the number of chances. But finally Emiliano [scored], he was composed in the box and scored.
"It was a good pass from Themba Zwane. He played for the team. He saw Emmie and gave him the pass."
'Jingles' believes his team must be at its best if they hope to overcome the Ivorian team in their own backyard in the return match next week.
"Asec will be difficult, we respect them, they have a star on top of their badge.You could see their number nine, quick and nippy. Mosa [Lebusa] and Ricardo [Nascimento] did well against him. He was the biggest threat," he added.
Sundowns are back in domestic action on Wednesday when they host Black Leopards in a league match at Loftus.