Kagawa was a central figure in Dortmund's back-to-back Bundesliga triumphs, but left Germany last summer when he opted to join Manchester United in a £17million deal.
Dortmund boss Jurgen Klopp claimed it broke his heart to see Kagawa leave, and then end up playing on the left wing rather than his preferred central position.
As he also suffered a significant knee injury that kept him out for nearly two months, the 24-year-old could be forgiven if he felt a rather big mistake had been made with his career.
Yet he insists watching Dortmund lose that all-German final against Bayern Munich at Wembley in May just made him more determined.
"I was proud to see my former team play in the Champions League final, and I was very excited for them - but it didn't make me feel I had made the wrong choice," he said.
"In fact it made me feel stronger because I want to be on the same field in future."
There were high points, in particular a hat-trick against Norwich in April.
But Kagawa summed up his feelings in a Japanese TV interview shortly after linking up with his team-mates for the latest leg of their five-match pre-season tour.
"It was a tough time for me with a lot going on in my head. I am not happy with my performance for the team at all," he said.