Ruotsalainen started his career in Kärpät. Nicknamed "Reksa" or "Rexi", he was recognized as a talented defenseman while playing for Kärpät junior teams. Kärpät was promoted to SM-liiga from 1. Divisioona for the 1977–78 season, and Ruotsalainen played 30 games and posted 23 points in his first SM-liiga season. During the 1979–80 season, Ruotsalainen and Kärpät played in the SM-liiga Playoffs, but were eliminated from the Finals by HIFK. Kärpät, however, won the bronze medal series, 2-0, against TPS. The 1980–81 season proved to be a breakthrough for Ruotsalainen. Kärpät made the finals against Tappara, coming from behind in the fifth and decisive game, winning the game 5-2. After his first Championship in SM-liiga, Ruotsalainen headed to National Hockey League (NHL).