I can:t really speak to how important Tomas Sandström was to the Rangers in the late-80s (probably very important), but I always thought he was key to the LA Kings' success in the c.1990-1993 period. He got injured all the time, but basically when he was healthy, the Kings were good, and when he was injured, they weren't particularly good.
The difference is most acute in the three 'full' seasons (i.e., seasons he wasn't traded within) tnat he played for LA, 1990-91, 1991-92, and 1992-93:
Overall, during these three seasons, the team was:
120-90-34 (.561)
But in the games that Sandström actually appeared in, the team was:
91-49-20 (.631)