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.
"I didn't score that much and I also had my injury.
"It was great we won the league. I just don't feel that I achieved everything that I wanted.
"For me personally, it wasn't a good season."
Kagawa clearly has a lot of affection for Dortmund.
Indeed, he claims he would like to return one day if the conditions were right.
However, he has important work ahead of him, proving to new manager David Moyes he is a top-class talent and just the man to step in for Wayne Rooney should the England forward get his wish to join Chelsea.
According to Moyes, predecessor Sir Alex Ferguson spoke "in glowing terms" of Kagawa.
But those words will mean little unless he matches them with deeds on the pitch.
"I have met the manager for the first time and we had a talk in the car on the way here," Kagawa said as he addressed the media ahead of tomorrow's encounter with Yokohama F Marinos.
"We still need to spend more time together to get to know each other.
"I am looking forward to building a relationship with him."
Though he is unlikely to feature for long, having done little training since the Confederations Cup ended a month ago, Kagawa will be the centre of attention given his obvious superstar status in his homeland.
Yet, for him, the biggest name on the pitch will be former Celtic midfielder Shunsuke Nakamura, who once scored a brilliant free-kick against United at Parkhead and has now moved back home.
"Nakamura is a very big global player and I am happy to be on the same pitch as him," Kagawa said.
"I would like to become the same global player one day.
"I will do my best for the team then hopefully it can happen."