Turkish international Kerim Frei joined Blues from Besiktas on 20 January 2017.
Frei, who plays on the left side of midfield, penned a three-and-a-half-year deal for an undisclosed fee.
Frei was born in Austria but grew up in Switzerland and played youth football for Swiss side Grasshoppers, before making the switch to England in 2010 to join Fulham’s youth academy.
His debut for the Cottagers came at the age of just 17 when he played in their Europa League fixture against Runavik.
He was later allowed to go out on loan to Cardiff City, where he made three appearances.
In total, during his time at Craven Cottage he made 33 appearances - 23 of which came in the Premier League.
After three years with the Londoners, Frei was snapped up by Besiktas for a reported fee of £3 million.
He subsequently went on to make 83 appearances and score seven goals for the Turkish giants.
Internationally, Frei has been a Turkish international since 2012 and currently has five caps for his country.