Simple but interesting analysis. As we know, utilisation of key players is significantly lower during pre-season games. It would be interesting to account for a structure of playtime. E.g. PSI (population stability index) could be computed for each team and season combimation to measure difference between minutes of each player played in pre-season and regular season games. This could be utilised as an additional variable that could somewhat capture this structural change. Obviously, this still would be a pretty naive analysis as there is a multitude of other factors to acccount for.