Arsenal have sold defender Krystian Bielik to Championship side Derby for a fee believed to be £10 million ($12m).
The 21-year-old, who joined from Legia Warsaw In 2015, leaves north London without having made a single Premier League appearance for the club.
“He is a young player at the age of 21 and it is a permanent deal which I think is a very good investment for the club. I am very happy he has joined the team," said Derby manager Phillip Cocu.
“He will get better and develop, but for his age he is a very good player.