Essentially, a player's worth is determined by the contribution to team goal differential, right? So wouldn't the best way to evaluate a player be to compare team goal differential with the player in the line-up, to the differential with the player out with injury? You would probably need multiple seasons to get an adequate sample size, but I think this method would show a players worth to a particular team very well.

Also, to compare the ES abilities of players who play different toughness of minutes, could one simply add opponents ES GD (goal differential) with a players ES GD? In other words, if player X plays opponents that have an average of +1 GD/hour, and he is +0.5 GD/hour while doing so, then isn't his ES worth +1.5 GD/hour? That player prevents 1 goal differential per hour, and creates 0.5 goal differential per hour.