Chelsea have signed Mateo Kovacic on a season-long loan from Real Madrid, with goalkeeper Thibaut Courtois expected to have his permanent move in the opposite direction confirmed later on Thursday.
Midfielder Kovacic, 24, has been at Real for three seasons since joining from Inter Milan.
He helped Croatia reach the final of this summer's World Cup in Russia.
"I am really happy and excited to be here. It is an amazing feeling," Kovacic told Chelsea's website.
"I will try to do my best for this club. It is a new league for me, the beginning is always tough but I am sure the coach and my new team-mates will help me and I am looking forward to a great season."
Chelsea director Marina Granovskaia added: "He is a perfect fit for us this season, his style is well suited to how Maurizio wants to play and we are sure he will prove to be a valuable asset for the club."
Kovacic joined Real in the summer of 2015 and helped the club win three Champions Leagues in a row in his three seasons at the club.
He is the second-youngest player to have ever scored in the Champions League after netting against Lyon at just 17.