There may not be such thing as an "average club" for any one league, but from Season X to Season X+1, can you rank the change of players for a club that stays in the top level (the Premier League and La Liga have 17 of these), the change of players for clubs promoted to the top level, and the change of players for clubs relegated out of the top level? One measure I can think of is minutes in common. Using only domestic league games, which is the same every season if the club stays in the league, I could look at each player's minutes with the club in the two seasons, take the smaller number for each player, and add it up for all the players. I could then divide the total by 37,620 to get a percentage. 37,620 = 11 players on the field at a time (if nobody has been ejected) multiplied by 90 minutes per game multiplied by 38 games per season. If a player was only one the club for one of the two seasons, the smaller number would be 0. There are several problems with this. A player could be employed by the same club for all of Season X and Season X+1 but play much more minutes in one of the seasons than the other due to missing time with an injury or gaining or losing playing time because of the quality of that player relative to other players who joined or left the club. Another problem is if promotion or relegation sends a club to a league that plays a different amount of games per season.