Hassinen was formed in the KalPa Kuopio organization. He joined HPK Hämeenlinna in 1992 and played his first pro game that season; by next, he established himself as a regular on the senior squad. He enjoyed his best season in 1996-97 with Hämeenlinna, scoring 19 times and adding 30 assists; he reached 49 points (24+25) once again in 2001-02 while playing with Tappara Tampere, a performance that granted him a spot on the All-Star Team. The next season, Hassinen joined HV 71 of the Elitserien for two solid campaigns abroad before returning home with HPK in 2004. He retired from competition in August 2007.