"At 21, he is a leader at Chelsea," he told Sport Bild. "It doesn't happen that often that a Bundesliga player moves to England and immediately convinces there.
"He quickly got used to the speed and the more physical style of playing in England. He is a role model for other U.S. players in the Bundesliga whose goal is the Premier League.
"Christian is perfect for Chelsea. All dangerous attacking plays go through him, he shines as a goal scorer and provides assists.
"If he can keep this level, he can play for the world's biggest clubs like Bayern Munich, Barcelona or Real Madrid."