It's obviously an unfair way to do it for players who play less than 90 minutes, but just work with me on this, mmmk?
*edit, okay, I figured out a slightly better way to do it by adjusting the points earned by the actual number of minutes played compared to the avergae number of minutes played. So it balances things out a bit more and looks a little more right. It also might help to think of the ratings less of a measure of how well did player X perform individually on a scale of 1-10 and think of it more as 'how much did player X being on the field help his team win'. So a higher rating indicates that the player was more instrumental in that team getting a good result. This way subs will generally be ranked lower, though it doesn't neccessarily mean they played poorly...