Real's Group D rivals also include English Premier League champions Manchester City [ Images ] and Dutch league title winners Ajax Amsterdam.
"This group is very tough. Some of them are a joke but this is THE group," Mourinho told reporters after the nine-times European champions went down to a goal in each half from Robert Lewandowski and Marcel Schmelzer.
Real are now second in their section with six points from three games, one behind leaders Dortmund who lifted the trophy in 1997.
Ajax are third on three points after defeating City (one point) 3-1 on Wednesday.
Some of the other groups include much smaller and weaker sides like BATE Borisov of Belarus and Denmark's Nordsjaelland.
"There is still a gap to the other two teams in our group," said Mourinho.
"At this moment we and Borussia are in a good position to go through but why can't City get 10 points? I think us and Borussia will have to think about getting 11."
Real's goal at Dortmund came from Cristiano Ronaldo [ Images ] in the 38th minute.
Photograph: Robin van Lonkhuijsen/United Photos/Reuters