OFFICIAL: Dortmund splash out on Diallo

The deal is said to be worth €28million, making Diallo their second-most expensive signing behind Andre Schurrle's €30million arrival in 2016.

The 22-year-old defender made 30 appearances last season for Mainz, scoring three goals and providing one assist.

The signing represents Dortmund's intent to change their personnel after a difficult campaign in 2017-18, in which Diallo is their fourth acquisition this summer.

He follows  Marwin Hitz (Augsburg), Marius Wolf (Eintracht Frankfurt) and Thomas Delaney (Werder Bremen), as well as the secural of new head coach Lucien Favre.

Diallo told Dortmund's club website: "After getting to know the Bundesliga in Mainz, I'm looking forward to the challenge of being able to prove myself at a club as big as Borussia Dortmund in the future. 

"Together with my team-mates I want to pay back the trust placed in me and contribute to BVB achieving its aims in every competition. I can't wait to get started with preparation for the new season under Lucien Favre."

While club sporting director Michael Zorc added: "He's a modern, intelligent defender, capable on the ball. He can play wide or in holding midfield and it was the combination of his potential and versatility that really caught our eye."

26 June 2018