The main thing to remember is that all of the recommendations are just that, recommendations. They are not perfect. With that being said, below describes the process of determining the recommendations for the defensive replacement hierarchies.
We first determine the Catcher recommendations. We take all position players on the major league roster for your team who played at least one game at Catcher in real life and order them by their (fielding grade*2)+(arm grade*3).
Next we determine the infield by taking all players who played at least one game at that position in real life and order them by their (fielding grade*3 + range grade*2).
Finally we determine the outfield by taking all players who played at least one game in the outfield in real life and order them by their (fielding grade*3 + range grade*2 + speed rating/12) [For speed, the divide by 12 is put in to make that value on the same scale as the grades]. All outfield recommendations will be the same.
Values for grades:
A+ = 12
A = 11
A- = 10
B+ = 9
B = 8
B- = 7
C+ = 6
C = 5
C- = 4
D+ = 3
D = 2
D- = 1