The young striker is in his first full season with Milan and currently Serie A's top goal scorer.
"He's gone beyond all our expectations. He turned 20 years of age last week and I do not know how many strikers of his age are scoring with this continuity," said Galliani.
"When he arrived from Padua he had a lot of problems. But now he is well and is one of the major talents in the world. His shooting is amazing and puts the ball where he wants with a great sense of goal.
"Stephan is a serious guy and mature. He seems to be older than his biological age. And he's well-liked by all, is serious and is a good guy in addition to being a great player."