The Turkey internationa has signed permanently with Fener this week.
Mourinho enthused: "I am very happy. I thank our president because he was one of the players I wanted to keep. Çağlar has qualities that I really like as a central defender.
"I already told him I wanted him to stay. In our team we always want players who want to stay in the club and who are committed. Before arriving at Fenerbahçe, I was interested in him when I worked at Roma, I wanted to bring him to the team, but it was not possible.
"He knows what I want and I know what he wants. He loves this place very much and wanted to move here permanently. Atlético Madrid had many options, but our club did an excellent job. Çağlar also made an effort to stay here. Tomorrow he will come with us to Austria.
"The retreat we will spend in Austria is very important, because we have many players in the European Championship, in which he was unable to participate due to the injury. So he will start working with us tomorrow. He will be very important for the team."