Lambert says the young striker's £7 million price tag still makes him feel "uneasy".
He said: "I think he can be huge, anything he wants to be," he said. "He can be a top, top striker because he is a handful.
"He will make mistakes because he doesn't know everything about the game but as time goes on, and he becomes experienced, he will be an absolute handful.
"He has come in and done great for us. He can plough the lone furrow because he does that for Belgium. It's the most I've paid for a player, yes, and it still makes me feel uneasy, but wait and see."