Mourinho thinks that the Bundesliga champions could provide Madrid with the sternest test of the three.
He told AS: "I'm not sure [if Manchester City are our biggest opponents], Borussia Dortmund have been champions of Germany twice in a row.
"They are well organised and very experienced, with many international-calibre players from Germany, and the top two from Poland. Their stadium and fans are impressive too."
Real begin their European campaign on Tuesday, September 18 when they host Premier League winners Manchester City.