"Most of our players reach their best when they are around 22 years old.
"But John is different. He played for the Under-19s when he was 17 and is used to being the youngest player.
"He is different to Van Persie, who is a very gifted player but less mature, and is the youngest of a new generation of players from Holland."
Van Zwam thinks John's childhood experience of fleeing war-torn Liberia with his mother and then settling in Holland must have forced him to grow up fast.
Fulham manager Chris Coleman is sensibly treating the striker carefully to allow him to develop fully and may not start him at Portsmouth tomorrow.
But Van Zwam is convinced John has the potential to become Holland's first-choice goalscorer. He said: "He is strong and fast, good with his head and both feet and has an eye for goal.
"I think he can still grow as a player and is a potential centre-forward for Holland in the next few years.
"If he continues to develop himself in the way he has done, he should get there."
Fulham have no new injuries for tomorrow's game but Sean Davis may be restored to the starting line-up after being dropped for breaking a club curfew.
