8 star player to a 9 star player?

Lets say I have a 8 Star player but I want to train him to a 9 star player etc etc...

There is now way of telling when or how he will turn into that 9 star player at least on the Mobile app..
How do I know what stat point I need in order to boost this player up to the next star rank..
The stars number is the average of the 22 skills levels.
Click on the player to see in details all attack, defense and physical skills.