The Brazillians needed an extra time winner from Keagan Dolly to progress in the competition, having failed to score against their NFD counterparts in the regular 90 minutes.
And Mosimane was far from happy with the performance.
"It was a tough game, they were strong. They were good, and they don't commit a lot of errors," Mosimane said.
"They pushed us to the limit. We didn't have many chances to score. But it was a cup game and we played it like that.
"What is good for us is that we have a good mentality to keep consistency in winning. The most important thing was to go through to the next round, and we did that."
Mosimane was also surprised by Highlander Park’s good showing, yet admitted he was disappointed in his own side’s display.
"I knew it will be tough but not like this,” he said. “We didn't play well. Most of our things didn't come out well. But what's important is even if we don't play well but we win, it’s okay."
The win sees Downs advance to the Last 16 of the competition as they look to defend the title they won last year.