public abstract class Player
{
    //fields
    private Sport _sport;

    //properties
    protected Sport Sport
    {
        get { return _sport; }
        set { _sport = value; }
    }
}

Player player = new FootballPlayer();
Sport sport = player.Sport;

protected Sport Sport { get; set; }

public Sport Sport { get; protected set; }

protected Sport Sport

public Sport Sport

public abstract class Player
{
    // Changed to auto property to save some key strokes...
    protected Sport Sport { get; set;}
}

public RealPlayer : Player
{
    public void Foo(Sport sport)
    {
        this.Sport = sport; // Valid
    }
}

var realPlayer = new RealPlayer();
realPlayer.Sport // Compilation error.