Havertz was superb in his first campaign as an Arsenal player.
But Hamann told In the Zone: "Kai Havertz had a decent end to his season with Arsenal. I've seen him since he was 17 years old and the kid has always been so good.
"When I watch him, I sometimes think he probably doesn't know how good he actually is. Even though he scored a few goals for Germany now, I just think he's capable of so much more. Havertz is not a centre forward and I don't think he wants to play as a centre forward. He's 25 and at a stage now in his career where he has to choose his position.
"He has to nail it down, because he's playing wherever Arsenal or Germany need him, and he plays there. If Arsenal bring in a centre forward, which they probably have to if they want to win the league, it'll be very interesting to see where Havertz plays or if he plays."