I just had a player of mine today, get labeled from starter to Key Player
What determines whether your player is labeled as Key Player or not?
If I recall correctly (and the hive mind will correct me if not!), it's based on the number of minutes played in league games. And since you've played the guy in question 94 minutes over the past two games, he's definitely a key player.
I usually see the tag "Key player" as a warning that a player is going to have GS problems because he's getting too many minutes. But that's just my view. ;)