Career
Lassen started his football career in Brødeskov near Hillerød, Denmark. He then moved to play for Farum BK, and at the age of sixteen, became the top scorer in the Junior League in Denmark. Moving to Football Club Nordsjælland, he played in their senior tram in the 2004-2005 season.
Before the start of the 2005-2006 season, Lassen transferred to Ølstykke.