Nordeus never gived a explanation about Phisical & Mental attributes,and the only attribute related to the "Mental" part imo can be fitness. So Can "Fitness" be related to this player form ? Like for example, a low fitness player ( even with great stats ) is "lazy" and have rating like 8-6-6-8-7 etc.. or a player with high fitness is a hard worker and have costant rating like 8-8-9-8-9 etc
Also in training section we can see this
This is a example, my 2 players with different fitness
1
2
Just a hypothesis