In his first professional season in 1975–76, Klassen, after a solid NHL training camp, commenced his professional hockey career with the Central Hockey League's Salt Lake Golden Eagles in the minor pro ranks. He played only four games in the minors before he was called up by California. Klassen made his NHL debut as one of two youngest players on the squad at the age of 19, along with Dennis Maruk, on October 8, 1975, at the Atlanta Flames. Klassen scored a goal in his first game. Klassen played the remaining 71 games for the Seals, racking up 6 goals and 15 assists, in his rookie year.