The midfielder won three trophies in two years at Westfalenstadion and would like to go back, but only once he has proven himself at Manchester United after a disappointing yearManchester United playmaker Shinji Kagawa says he would love to return to Bundesliga outfit Borussia Dortmund later in his career.The 24-year-old struggled with injuries in his debut season at Old Trafford, scoring six goals in 26 appearances across all competitions.The Japan international was outstanding for Dortmund in the 2011-12 campaign, helping them to a league and cup double, but largely failed to replicate his form in England and admitted he would like to return in the future.Kagawa told TV Tokyo: "Yes, one day I will [return to Dortmund]. If Dortmund are OK with it. I'm happy we won the league [last year] but I'm not satisfied with my performance."The club has a long history and it was great that I could play in Sir Alex Ferguson's last season. At first there were also moments when I felt a bit lost but I tried not to give too much importance on such a feeling."Kagawa has described the 2013-14 season as "really important" for himself, as he bids to become a regular starter under new United boss David Moyes.But competition for places is high, with Kagawa – who was often employed on the wing last season – fighting for a spot alongside Ashley Young, Antonio Valencia and new signing Wilfried Zaha."I have no excuse any more after a year in the Premier League," he said."I know I'm at a place where you can't play if you don't achieve results. I'm determined to do it, also because there will be World Cup. It's really an important year for me."