The Brazilians missed out on an opportunity to add another continental trophy to their collection, with the Algerian side producing a strong second-half display to claim the Super Cup at Loftus Versfeld Stadium on Sunday.
Sundowns were dealt an unexpected blow even before kick-off when Nuno Santos suffered an injury during the warm-up, forcing Cardoso to make an adjustment to his starting line-up, with Cassius Mailula coming in as his replacement.
The visitors took the lead through Zakaria Draoui, but Iqraam Rayners converted from the penalty spot to bring the Tshwane giants back into the contest before half-time.
However, Sundowns were unable to maintain their momentum after the break, conceding twice as USM Alger secured a convincing victory.
Cardoso admitted that his side made too many mistakes, particularly when defending, but also felt the condition of the Loftus pitch made it difficult for his players to execute their usual possession-based approach.
"Of course, there are a few mistakes, a lot of mistakes mostly on the goals we made mistakes. I think we were strong while we tried to resist, but then we could not. Second half with the pitch it was very difficult to play," he told SABC Sport after the game.
The Portuguese coach also revealed how losing Santos shortly before the match disrupted his preparations, especially after having to make an early substitution.
"It's not easy to come with on the warm-up, prepare the match and suffer a loss of a player that is a key player after ten minutes you lose a striker and have to make another sub," he explained.
Despite his disappointment, Cardoso apologised to the Sundowns supporters, club leadership and board for failing to deliver the trophy.
"I'm very sorry for our fans, they were here today (Sunday), we wanted to give them joy for our chairman, board and I feel a lot of pain today because of that," he added.