Kagawa, who enjoyed two seasons at Westfalenstadion, is tipping his old club to triumph this weekend.
The Japanese star said: "I want to play better in the Champions League next season because I've been inspired to see Dortmund make it to the final.
"I was encouraged to know there is a chance for me to be on the big stage with United.
"I've followed how they are doing and have kept in touch with some of the players. Each time they progressed in the Champions League, I sent them a text to say congratulations.
"Bayern will find it hard against Dortmund as they haven't beaten them in the league for three seasons.
"Dortmund will play hard without being over-confident. I expect it will be an exciting match and I hope Dortmund will be European champions."