The Nigeria striker has been at the club since the age of eight, before making his breakthrough into Arsène Wenger’s side last season.
Jonker says the 20-year-old’s drive and determination is an example for all to follow.
“What makes [Alex] different from many other boys is that, when you analyse him, he’s good at all aspects,” he told Arsenal Player. “It’s not just him being a good player, he’s also a stable guy who’s working hard every day. He’s not getting distracted by having a lot of success.
“When I came in, I remember he was about to go on loan after having a knee injury and he was not that fit and not able to play 90 minutes. Arsène put him on a training pitch and he did quite well. Then his development started and he became some sort of a rising star.
“He played better and better in the under-21s, played quite well in the under-19s and scored two brilliant goals against Bayern Munich, and then he deserved his chance. He was named on the bench [for the first team] and then Arsène gave him his chance and he was stable.
“He didn’t panic, he didn’t have any wrong expectations. He stayed in his shoes. He kept working hard and grabbed his chance. We respect him a lot and I’m very pleased with his performances.
“He’s the example of a good player who has not been identified by anybody as a potential first-team player. Many, many people seem to think you have to recognise an under-nine as a potential first-team player. When you spend a long time in youth football, you understand that you will never know.
“There are positive and negative surprises, but Alex is one of those boys who has continued to work hard, and kept his commitment and dedication no matter what has happened. In the end he made it and he has been a big surprise to everyone at Arsenal but we all love it. He’s surprised us all.”