Celtic defender Dedryck Boyata will join side Hertha Berlin this summer.
The 28-year-old centre-back's contract expires at the end of the season, although Celtic manager Neil Lennon said last month he was hoping a new deal would be agreed.
Boyata - who was part of Belgium's World Cup squad last summer - has not played since March because of injury.
"If it is the case, it's a big blow for us because he's been superb since I've been here," Lennon told BBC Scotland.
"I only became aware of the story on the way in. I saw him this morning training the gym at Lennoxtown but I didn't get a chance to speak to him."
Boyata said he "glad" to join a Hertha side that finished 11th in the Bundesliga this season.
He added: "The club is ambitious and has goals that I stand by and I was very impressed by how Michael Preetz tried to talk to me and showed me a clear plan."
Signed from Manchester City in 2015 during Ronny Deila's tenure, Boyata has made 125 appearances for Celtic.
"We've had Dedryck in our sights for some time and are fully convinced of his qualities," said Hertha general manager Michael Preetz.
"We are confident that he is a real reinforcement for our defence."
Celtic also face losing another centre-back, Filip Benkovic, this summer when the 21-year-old's loan deal from Leicester City ends.
There has been reports that Brendan Rodgers could allow the Croat to return to Scotland for next season and Lennon was open to that idea.
"He's a very talented player and he's been unlucky to be out of the side of late," he said. "He'd be one we'd love to get back next year."