Ruotsalainen was signed by the New York Rangers of the NHL in 1981. During his debut season, he scored 56 points in 78 games. Initially paired on defense with Barry Beck, Ruotsalainen was eventually moved to forward. Coach Herb Brooks said Ruotsalainen was the best skater in NHL. Ruotsalainen's fluid skating helped him compensate for his lack of size. Ruotsalainen had his NHL-career high in points during the 1984–85 NHL season, leading the Rangers in scoring with 73 points in 80 games. Ruotsalainen played in the 1986 NHL All-Star game.