The latest Manchester United transfer news, views and rumours.
Indeed, it reportedly took the Japan international just two minutes to score his first goal in training after completing a switch back from Manchester Untied to the Bundesliga side.
And, according to the London Evening Standard, Dortmund coaches have been 'amazed' by how quickly Kagawa has adapted to the German side's methods.
The club's coaching staff are also apparently shocked that Manchester United could not get the best out of the 25-year-old.
Sir Alex Ferguson
The club have forked out a record fee on Angel Di Maria and have also recruited the likes of Luke Shaw, Daley Blind, Ander Herrera and Radamel Falcao this summer.
Asked what he thought of United's transfer window, Ferguson told reporters: "Certainly it's amazing, the amount of money spent nowadays."
United are said to hold an interest in the 21-year-old but he will reportedly sign a new five-year deal with the Ligue 1 side.