The Japan international enjoyed a strong first season in England as he claimed six goals and six assists in 26 appearances as United romped to the Premier League title.
However, Sir Alex Ferguson retired at the end of the campaign and things started to go wrong for Kagawa under David Moyes before his successor Louis van Gaal made it clear he wouldn't be playing much at Old Trafford if he stayed.
“There was another change of manager [when Moyes was sacked]," Kagawa told The Athletic. "I was now in my fourth manager in just over two years," he added. "I joined a club with the same manager for 25 years and then they had Moyes, Giggs and van Gaal.
“I did not play well in the World Cup finals in Brazil. Japan were bottom and eliminated. I felt I needed a new challenge to help me get back to my best. Maybe that could have been in Manchester, but then Angel Di Maria and Radamel Falcao arrived.
"That only made my situation worse. I knew it was time for me to leave Manchester after two years when Van Gaal said to me: 'Shinji, you are not going to play as much for me.'"