It seems demanding for a server handling thousands or millions of teams to check the formations we used previously.

To determine the player needed, the system may compute the percentage that different positions appearing in the 4 selected formations, and then compare with current squad to find position in shortage.

However, it is not easy. Even another human cannot fully understand what other persons think. For example, if I often use one striker, this can mean (1) I prefer 1 striker formation, or (2) I have no choice because I lack strikers for rotation.