But Ballack told Sport1: "One thing is clear - Havertz is not yet a finished player.
"That is not meant negatively, but I still see potential for improvement in his personality development and body language.
"But that is completely normal at his age. As a captain, he first has to learn to deal with this responsibility.
"His style of play has always been very carefree. If he ever lost the ball, he was allowed to stop. With a top club, everything will be more difficult and will no longer be accepted.
"He must and will be aware that he has an exceptional position in Leverkusen. He should ask himself now, 'How long do I want to have this? How long do I enjoy it? Or will I take on the next challenge with even greater demands on my game this summer?'."