The Brazil international was said not to have been interfering with play when his team-mate put the Bavarians a goal ahead with just 17 minutes of the game gone
Ribery fired the hosts ahead when he drilled the ball past Casillas some 17 minutes into the game, but television replays showed that Gustavo had been standing directly in front of the Spain goalkeeper.
The officials had felt that the Bayern midfielder had not been interfering with play but Casillas claimed afterwards that his view had been obstructed by the Brazil international.
"Yeah, the position he was in annoyed me," the World Cup winner is quoted as saying by his club's official website. "He obstructed my view of the ball.
"You might not notice it at first because it happened very quickly, but it is true that he was offside."
Madrid managed to draw level through Mesut Ozil early in the second half but Mario Gomez scored in the final minute of normal time to secure a 2-1 victory for Bayern.
"We managed to create a few more chances in the second half but ultimately they have scored at the end, so everything will now be decided in Madrid," he mused.
"We have scored away and that's important. But we must remember that Bayern have good players."
However, before the second leg, Madrid face a crucial Clasico clash with Barcelona at Camp Nou this weekend.
Many pundits believe that Los Blancos, who top La Liga by four points from the Blaugrana, will all but seal the title if they triumph in Catalunya, but Casillas does not see it that way.
"In Barcelona we will try to win and do our best to look after our own interests," he explained. "It will be an important game for both teams, but not decisive."
Madrid's meeting with Barcelona kicks off at 20.00CET on Saturday, while Bayern are due in the Spanish capital on Wednesday week.