Jurgen Klopp says his "heart breaks" to see Shinji Kagawa's game time reduce significantly since moving from Borussia Dortmund to Manchester United.
Kagawa made the switch to Old Trafford last summer, but has not enjoyed a regular starting spot. The Japan has also been used in a wide position by Sir Alex Ferguson, and Klopp believes his considerable talents have thus far been wasted.
Klopp told the Guardian: "Shinji Kagawa is one of the best players in the world and he now plays 20 minutes at Manchester United - on the left wing!
"My heart breaks. Really, I have tears in my eyes. Central midfield is Shinji's best role. He's an offensive midfielder with one of the best noses for goal I ever saw.
"But for most Japanese people it means more to play for Man United than Dortmund. We cried for 20 minutes, in each others' arms, when he left.
"One year before that Nuri Sahin went because Real Madrid is the biggest club in the world. If players are patient enough we can develop the team into one of the biggest in the world."
His side's run to the Champions League final has brought Klopp to the attention of some of the world's biggest clubs, but for now he is happy at Dortmund.
"In this moment I don't think of anything else," he said. "If I went to many clubs now and said: 'Hello - bring me offers', maybe some would start running. But I'm not interested because, for me, this is the most interesting football project in the world.
"In three or four years, if someone wants me, we can speak. But, for now, this is the best place for me."
Information from the Press Association was used in this report.