Defensive stats for players with less than (let's say) 10 games at a position should be give a generic (and low) defensive value at that position. They haven't had enough chances to establish a reasonable defense level and can have GOLD GLOVE type performance. For RT leagues this might need to be turned on after the first 25 games or so, or have a graduated index as a % of all games played to date.

e.g. Sweeney at C a few years back
Glaus at SS for the last 2 years at least