Jurgen Klopp is 'heartbroken' at Shinji Kagawa's bit-part role at Manchester United and says the midfielder is one of the world's best players.
The 24-year-old moved to Old Trafford from Borussia Dortmund last summer, but has been far from a first-team regular and even then has often had to play out of position.
The departure of Kagawa is part of a pattern for Dortmund coach Klopp, who this season has seen Mario Gotze agree to join Champions League final opponents Bayern Munich, with Robert Lewandowski looking likely to follow.
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.
He said: "In this moment I don't think of anything else.
"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."
Meanwhile, former Dortmund and Bayern coach Ottmar Hitzfeld believes Jurgen Klopp will be the coach of Bayern at some point in the future.
Dortmund coach Klopp has built a strong reputation with the Westphalians, leading them to back-to-back Bundesliga titles and a place in Saturday's Champions League final.
His name is now being linked with every coaching vacancy at Europe's top clubs, and Hitzfeld, who led both Dortmund and Bayern to Champions League glory, expects Klopp to make the same move he made.
"I can imagine Klopp coaching Bayern at some point," he said.
"Bayern nearly always appoint the best German coach. Jurgen Klopp was incredibly successful with Mainz and he is now doing a great job with Dortmund."
Klopp was already close to becoming Bayern's coach when he left Mainz in 2008.
Before he became coach of Dortmund, he had already held talks with Bayern president Uli Hoeness, and reportedly reached an agreement until Bayern chairman Karl-Heinz Rummenigge vetoed the move and instead appointed Jurgen Klinsmann as coach.
"I let myself be convinced that we should try the adventure with Klinsmann," said Hoeness to the Suddeutsche Zeitung newspaper in 2011.
"I had already reached a verbal agreement with Klopp and I then had to go back on our word when we appointed Klinsmann.
"We now know how big a mistake that was."
Hitzfeld nevertheless believes it is a mistake they will not make again, should Klopp become available when his contract with Dortmund expires in 2016.
"He is a highly intelligent coach," said Hitzfeld to Sport1 television. "He lives for football and can set emotions free.
"He really motivates his team and sets them up perfectly tactically."
A move is not foreseeable in the near future, though, with Bayern appointing Pep Guardiola on a contract until 2016 from next season, but Klopp, who was already on Bayern's radar once, could still be there if the Spaniard is unable to successfully fill Jupp Heynckes' shoes next term.
