The Brazilians crashed out in the second preliminary round against Angolan outfit Wiliete SC, missing out on the group stages for the first time since 2015 despite entering the competition as defending champions.
Cardoso was left disappointed by the result, particularly because he felt Sundowns had enough opportunities to settle the tie but failed to capitalise on their dominance.
Speaking to SABC Sport after the match, the Portuguese coach suggested that Wiliete offered little attacking threat, with Sundowns effectively handing their opponents an opportunity through their own mistake.
"It's obviously difficult to express anything when you don't qualify for the group stages and you cannot even proceed in the competition and defend the title," he said.
"But we just can look at ourselves. I think besides one shot from the first half there is nothing from Wiliete in this match.
"The goal they score is a pass from our own striker. We had so many chances, we can only complain on us, it's a very tough night, very difficult moment."
The Sundowns coach also identified his players' reaction after conceding as a major concern, explaining that they allowed frustration to affect their decision-making instead of remaining composed with sufficient time left to recover.
"We can only complain on us on what we could have done, the game was so simple to close it but we let the energy not go in the right direction and after we suffered the goal still 40 minutes to play and we let emotions take control of us and not play in the most clever way," Cardoso added.
The elimination represents a significant setback for Sundowns, who had been hoping to continue their continental title defence.
However, Cardoso insisted that dwelling on the disappointment would not help his team, with the focus now shifting to the remaining competitions.
"It's a very tough result, disaster is another thing in life. The only way is to move forward. There is competitions to win, very quickly we have to turn over," he said.
"Obviously it's a tough one and fortunately a lot of losses we had, but nothing to complain. Just look forward and win."