Here Jan, working his way back to fitness after an ankle injury, tells us how that friendship blossomed...
Where did your friendship with Mousa start?
Jan: "I first met him when I was 12. I went to Antwerp to play for a team (Germinal Beerschot) and he played for the same team. I played for that team until I was 16. We weren’t really friends then, we knew each other and you know, he’s a good guy.
"I then saw him again in Amsterdam. When I was 16 I moved to Amsterdam to play for Ajax and when he was 18 or 19, he came to Amsterdam to play for AZ Alkmaar, which is close to Amsterdam.