This unlocked for me once i reached 83 overall, i was 83 and 1/4 attacker, 82 and 3/4 midfielder and 82 and 1/2 defender, to change position go to skill tree and hit Y.
This will take a long time and many games...thanks to darzo93mufc the quickest match you can replay seems to be rio de janiero 3v3 rush first to x (first to 3 goals) once you reach the high 70's you will gain progress every 3 or 4 games but this is much quicker than 6 minute games
Thanks to futher information in the comments this can unlock differently for people, between 82 and 84 seems to be the numbers and you will receive a message on screen stating you've maxed out your overall rating. Changing positions every so often also helps to trigger stats and progression again..Good luck
If anybody want's to add anything or if there is a quicker way leave a comment and i will update, thanks