 Nielsen has attracted controversy at various times in his career for falling into serious gambling debt, assaulting a teammate and then, in November 2008, admitting matchfixing in a Danish game played in 2004.Born in Skagen, Nielsen started playing youth football in Skagen IK and Frederikshavn fI.He scored 17 goals in 24 matches for the Danish under-16 and under-17 national teams.