He produced his best haul for Chelsea with 12 Premier League goals last season and has already knocked in four more this term.
"I saw he was a striker, not someone in midfield or in defence," said Ancelotti. "He was a player with an eye for goals who needed convincing he could do just that.
"I moved him forward and since then he has blossomed into a world-class player.
"Now he is playing in the right position. In the past maybe he was used too far back and he was unhappy. Now he is more free, he can show his abilities."