I think as long as he's playing he has the potential to be very very good. Playing a really significant number of games from now till 23-24 is crucial if he wants to turn into a really top notch player. Wrong choice and he'll be a bit part player and quite likely not progress beyond where he is now.
I do hope that Chelsea sell to Everton though, working well with Martinez, seems to have added a few things to his game and frankly just like seeing Martinez do well with Everton. For Chelsea as above, a cheaper deal would give them a bit of negotiating power in terms of clauses to maybe get him back in the future and cheaper deal would obviously suit Everton.