Borussia Dortmund boss Jurgen Klopp has expressed his surprise that Manchester United sold Shinji Kagawa this summer.
The Japanese international signed for Manchester United from Dortmund in the summer of 2012 but endured two difficult seasons at Old Trafford.
He played under three managers, Sir Alex Ferguson, David Moyes and Louis van Gaal and Klopp believes this hindered his development.
"I was kind of surprised Kagawa was on the market again," Klopp said as reported by Goal.com.
"His first year at Manchester United was Sir Alex Ferguson's last year, so that isn't the time to develop players. The only thing you want at that stage is results and titles.
"Kagawa's second year at United was the first year of the new coach, so that's also all about results. Then there was a third manager with a very successful philosophy [van Gaal].
"So we saw the chance to sign Kagawa and we took it. Kagawa was sensational against Freiburg."
He marked his Dortmund return with a debut goal for the club against their Bundesliga opponents, which was tough news to take for United fans that had such high hopes for Kagawa when he signed for the club.
He never failed to settle and the writing was on the wall when Moyes splashed out £37m to sign Juan Mata last January. Even his place is now under threat at United, as new boss van Gaal has invested further in the attacking midfield position, signing Angel Di Maria from Real Madrid.
The Argentine enjoyed a wonderful home debut for United, scoring and assisting against Queens Park Rangers in a 4-0 victory on Sunday afternoon.
Man United transfer news
Klopp leads his Dortmund team against Arsenal in the first round of the Champions League group stage this evening. The Dortmund boss praised Arsenal's transfer business, and believes the club now has 'great stability' and is able to play the style of football Arsene Wenger wants to play.