A place in the last four of UEFA's elite club competition for the first time in a decade was sealed by a 2-1 triumph at the Etihad Stadium on Tuesday night.
Liverpool and Roma are the two sides currently in the hat for the next round, but Van Gaal expects Real Madrid and in particular Bayern Munich to make their quality count if they also progress through from their last-eight ties.
"In comparison to the other teams, especially Real and Bayern, [Liverpool and Roma] have less individual quality," he told Dutch TV station Ziggo Sports.
"In principle they are not candidates, but they are a good team and that is always dangerous. If I have to bet now, I would put my money on Bayern Munich, which in my opinion has the best defence and a coach that adapts his team to the opponent, but who also assumes his own qualities, which is the key to success."