Amount of greens to upgrade 1 player 7 star to 8 star can be used to upgrade 2 players 6 star to 7 star. Dont put all eggs in one basket. Best is to have several key players. If he is that good, possible he couldn't get any better when he is 8 star. There might be a day where he cant perform and coincidently its a cup final.

2 seasons ago I was in a league full of title winners, 6-7 of them all won league previous season. Same quality. And I basically finished with 10 pts to spare, and guess what, none of my players featured in league goals, form and assist ranking.