"They had a lot of set pieces and I thought we were very aware whereas, in the last game, the same thing that (coach) Benito (Floro) told us to look out for we got beat early.
"I thought, defensively, we improved in that respect."
After a scoreless first half in which Canada defended well, AC Milan winger Valter Birsa scored for Slovenia early in the second. His low kick got by diving keeper Kenny Stamatopoulos in the international friendly.
"We really limited their chances, to be fair," said De Rosario.
"Good teams create their luck and that little touch to the side that he hit into the far post, Kenny was a little bit to the left and Adam (Straith) was a little off his man, but it's all part of improving, it's all part of learning."
Floro, who took over the national men's team in August, saw aspects of their game that need work.
"Our wingers, forwards and midfielders need to understand perfectly that it's not about having the ball, but we need to start passing the ball behind our opponents' backs," he said. "We are always playing the ball in front of them and it is very difficult to score from that."
De Rosario had what looked like the best shot in the first half to snap a scoring drought that has now extended to 10 games.
"Their goalkeeper made a wonderful save off me, you couldn't do anything more about that," he said.
Canada created pressure and a few scoring chances in the final minutes of the second half but couldn't convert any into a goal. De Rosario said it was encouraging to see, nonetheless.
"The positive that I like to see is that, towards the end, we're still pushing, we're still going forward, our heads are not going down and for me, that's huge."
Like Canada, Slovenia didn't qualify for the 2014 World Cup. But the two teams are nowhere close in FIFA ranking with Slovenia at 30 and Canada at 111.
While their defence appeared stronger than their last game against the Czech Republic, Floro said that also needs work.
"Pressing is an important part of our defensive system but it's not enough," he said. "We need to press and tackle.
"The pressing is good. They understand that it is necessary to press to prevent the opponent from passing the ball but the next step is to steal the ball. We need to tackle better and fight for position better."
De Rosario says it will take time.
"It's all one step at a time, the next camp, the next game," he said. "We're not getting ahead of ourselves. This year was a very difficult year. No one hates losing more than me but, when you lose, you're learning and you move forward."