 After joining Hammarby halfway through the 2003 season, the 18-year-old Andersson made two competitive appearances for his new side the same year.His debut in Allsvenskan came against Örebro SK on 20 October in a 1–1-draw, coming on as a second half-substitute replacing veteran midfielder Christer Fursth, who coincidently played the last game in his professional career.Ahead of the 2004 season, Hammarby and manager Anders Linderoth promoted several other youngsters to the senior squad on permanent basis; Björn Runström, Fredrik Stoor and Erkan Zengin.