Chelsea have completed the signing of former midfielder Nemanja Matic from Benfica for a fee believed to be in excess of £20m.The Serbia international confirmed his departure from Portugal by announcing that last weekend's win over Porto was his final game for the club. He told A Bola: "I want to thank everyone at the club and all the fans. Benfica will forever be in my heart. "I played here for two-and-a-half years, gave my best at every training session, and now it is the right moment to leave. "I want to improve, I want to play in a better league and I want to go to Chelsea, a club I also love. "I would also like to thank Benfica's president, he did everything he could to keep me, but I wanted to leave. "I insisted on leaving because I think this is the right moment."